Non-Mendelian Inheritance

Complex Patterns of Complex Patterns of InheritanceInheritance

Many things can happen to Mendel’s Many things can happen to Mendel’s factors during the process of meiosisfactors during the process of meiosis MutationsMutations cross-over between homologues cross-over between homologues non-disjunction. non-disjunction.

Complex Patterns of Inh’ce Complex Patterns of Inh’ce (cont’d)(cont’d)

There are also exceptions to the There are also exceptions to the postulate that factors occur in pairs postulate that factors occur in pairs and the law of dominance, as well as and the law of dominance, as well as the law of independent assortment—the law of independent assortment—it is now known that many genes are it is now known that many genes are linked on the same chromosome. linked on the same chromosome.

Linked traits are genes that are Linked traits are genes that are located on the same chromosome.located on the same chromosome.

Increasing Genetic Increasing Genetic VariabilityVariability

Recall: Recall: Cross-over in MeiosisCross-over in Meiosis During Meiosis I, the chromatids in a During Meiosis I, the chromatids in a

tetrad pair are so tightly aligned tetrad pair are so tightly aligned together that the non-sister together that the non-sister chromatids from homologous chromatids from homologous chromosomes actually exchange chromosomes actually exchange genetic material in a process known genetic material in a process known as crossing over. as crossing over.

This further shuffles the ancestral This further shuffles the ancestral genes so that a single chromosome in a genes so that a single chromosome in a gamete may have genes from both the gamete may have genes from both the maternal and paternal ancestors. maternal and paternal ancestors.

Crossing over can occur at any location Crossing over can occur at any location on a chromosome, and it can occur at on a chromosome, and it can occur at several locations at the same time.  several locations at the same time. 

It is estimated that during meiosis in It is estimated that during meiosis in humans, there is an average of two to humans, there is an average of two to three crossovers for each pair of three crossovers for each pair of homologous chromosomes.homologous chromosomes.

Increasing Genetic Increasing Genetic VariabilityVariability

Increasing Genetic Variability Increasing Genetic Variability (cont’d)(cont’d)

The chromosome pieces further away from The chromosome pieces further away from the centromere cross over most frequentlythe centromere cross over most frequently; ; the frequency diminishes as the the frequency diminishes as the centromere is approached. centromere is approached.

There can also be multiple cross-overs. There can also be multiple cross-overs. They may occur because the genetic code They may occur because the genetic code

on each section of the chromatid is similar.on each section of the chromatid is similar.

Increasing Genetic Variability Increasing Genetic Variability (cont’d)(cont’d)

Sex-Linked TraitsSex-Linked Traits (p 318-320)(p 318-320)

Some patterns of inheritance seem Some patterns of inheritance seem to weigh more heavily in malesto weigh more heavily in males I.e. more males than females have I.e. more males than females have

hemophilia, red-green colour blindness, hemophilia, red-green colour blindness, Duchenne muscular dystrophy and Duchenne muscular dystrophy and others. others.

How this was discovered:How this was discovered:

In 1910 In 1910 T. H. Morgan studied the T. H. Morgan studied the Drosophila fly Drosophila fly and found a mutant and found a mutant male fly, which expressed the trait of male fly, which expressed the trait of white eyes instead of the normal red white eyes instead of the normal red eyes. eyes.

This trait was very unusual in that This trait was very unusual in that species and Morgan wanted to see if species and Morgan wanted to see if the trait would be passed on to its the trait would be passed on to its offspring. offspring.

He experimented to He experimented to find if this strange find if this strange trait would be trait would be inherited according inherited according to Mendel's research. to Mendel's research.

First he crossed the First he crossed the mutant male fly with mutant male fly with a normal female with a normal female with red eyes, to observe red eyes, to observe whether the white or whether the white or red eyes were red eyes were dominant. dominant.

Sex Linked Traits Sex Linked Traits (cont’d)(cont’d)

The F1 generation all had red eyes, The F1 generation all had red eyes, which made Morgan conclude that red which made Morgan conclude that red eyes were dominant over white. (eyes were dominant over white. (See Fig See Fig 12.11 p 319)12.11 p 319)

He continued the steps of Mendel's He continued the steps of Mendel's experiment by crossing two flies from experiment by crossing two flies from the F1 generation with each other. the F1 generation with each other.

Out of Out of 4252 4252 flies in his F2 generation, flies in his F2 generation, 782782 had white eyes but surprisingly all had white eyes but surprisingly all the flies with white eyes were also male. the flies with white eyes were also male.

Sex Linked Traits (cont’d)Sex Linked Traits (cont’d)

This strange observation puzzled This strange observation puzzled Morgan to wonder why there weren't Morgan to wonder why there weren't any females with white eyes. any females with white eyes.

He then crossed flies from the FHe then crossed flies from the F11 generation with the original male fly generation with the original male fly with white eyes. with white eyes.

This cross resulted in white-eyed and This cross resulted in white-eyed and red-eyed males and females, making red-eyed males and females, making a 1:1:1:1 ratio.a 1:1:1:1 ratio.

Sex Linked Traits (cont’d)Sex Linked Traits (cont’d)

We see this pattern in humans—in We see this pattern in humans—in hemophilia, and red-green colour hemophilia, and red-green colour blindness.blindness.

Why? How does it specifically affect Why? How does it specifically affect males more often than females?males more often than females?

Sex Linked Traits (cont’d)Sex Linked Traits (cont’d)

Sex-Linked Traits (cont’d) Sex-Linked Traits (cont’d)

If a gene is found only on the X If a gene is found only on the X chromosome and not the Y chromosome and not the Y chromosome, it is said to be a chromosome, it is said to be a sex-sex-linked traitlinked trait. .

Because the gene controlling the Because the gene controlling the trait is located on the sex trait is located on the sex chromosome, sex linkage is linked to chromosome, sex linkage is linked to the gender of the individual. the gender of the individual.

Usually such genes are found on the Usually such genes are found on the X X chromosomechromosome. The Y chromosome may be . The Y chromosome may be missing such genes (See Diagram above.). missing such genes (See Diagram above.).

Sex Linked Traits (cont’d)Sex Linked Traits (cont’d)

The result is that females will have two The result is that females will have two copies of the sex-linked gene while copies of the sex-linked gene while males will only have one copy of this males will only have one copy of this gene. gene.

If the gene is recessive, then males If the gene is recessive, then males only need one such recessive gene only need one such recessive gene to have a sex-linked traitto have a sex-linked trait rather than rather than the customary two recessive genes for the customary two recessive genes for traits that are not sex-linked. traits that are not sex-linked.

This is why males exhibit some traits This is why males exhibit some traits more frequently than females.more frequently than females.

Sex Linked Traits (cont’d)Sex Linked Traits (cont’d)

Showing sex-linked Punnett Showing sex-linked Punnett Squares Squares

Red-Green Colourblindness:Red-Green Colourblindness: Because the allele is linked to the X Because the allele is linked to the X

chromosome, we show it as a chromosome, we show it as a superscript on the X:superscript on the X:

Eg. XEg. XCC normal vision gene normal vision gene

XXcc colour blind genecolour blind gene

Children who inherit Children who inherit this trait have this trait have difficulties with green difficulties with green hues, usually seeing hues, usually seeing them towards a “red” them towards a “red” spectrum. spectrum.

Reds tend to be seen Reds tend to be seen darker, and in low darker, and in low light colour light colour differentiation is differentiation is difficult—both difficult—both appearing black to the appearing black to the person.person.

Showing sex-linked Showing sex-linked

Punnett SquaresPunnett Squares

The Y chromosome does not carry this The Y chromosome does not carry this trait at all. trait at all.

A normal-vision mother whose father A normal-vision mother whose father was colour blind has a child with a was colour blind has a child with a normal-vision man. normal-vision man.

What is the probability the couple will What is the probability the couple will have a child with colour-blindness? have a child with colour-blindness?

What is the probability a son born to What is the probability a son born to them will be colour blind?them will be colour blind?

Eg. In humans, red-green Eg. In humans, red-green colourblindness is a recessive trait colourblindness is a recessive trait

located on the X chromosome.located on the X chromosome.

XXCCXXcc x X x XCCYY XC Y

XC

Xc

Genotypic Ratios:

Phenotypic Ratios:

• Notice, Y doesn’t have this gene!

XC XC XCY

XC Xc XcY

1:1:1:1

3:1

HemophiliaHemophilia Hemophilia is a term Hemophilia is a term

that covers a wide that covers a wide variety of clotting variety of clotting disorders—some disorders—some clotting factor(s) are clotting factor(s) are missing or are missing or are defective, resulting defective, resulting sometimes in sometimes in uncontrolled uncontrolled bleeding.bleeding.

Hemophilia is a recessive sex-linked Hemophilia is a recessive sex-linked trait on the X chromosome.trait on the X chromosome.

What are the chances a couple will What are the chances a couple will have a daughter with hemophilia if have a daughter with hemophilia if the mother is a probable carrier and the mother is a probable carrier and the dad has a form of hemophilia?the dad has a form of hemophilia?

What is the chance they would have What is the chance they would have a child with hemophilia?a child with hemophilia?

HemophiliaHemophilia

XXHHXXhh x X x XHHYY Xh Y

XH

Xh

Genotypic Ratios:

Phenotypic Ratios:

• Notice, Y doesn’t have this gene!

XH Xh XHY

Xh Xh XhY

1:1:1:1

1:1:1:1

Sex-Influenced TraitSex-Influenced Trait

These are These are not true sex-linked not true sex-linked because it is not on the X nor Y because it is not on the X nor Y chromosomeschromosomes, but because of , but because of hormones or other such differences hormones or other such differences between genders, these traits show up between genders, these traits show up and “look” like sex-linked traits. and “look” like sex-linked traits.

For example, in male-pattern-baldness is For example, in male-pattern-baldness is a dominant trait in males but recessive a dominant trait in males but recessive in females. in females.

Male Pattern BaldnessMale Pattern Baldness

For example, male-pattern-baldness For example, male-pattern-baldness is a dominant trait in males but is a dominant trait in males but recessive in females. recessive in females. Male heterozygotes will go bald. Male heterozygotes will go bald. Female heterozygotes will not go bald. Female heterozygotes will not go bald.

A female would need to inherit the A female would need to inherit the trait from both parents to lose her trait from both parents to lose her hair.hair.

We would do a standard punnett We would do a standard punnett square forsquare for a dihybrid cross: a dihybrid cross:

A female with a lot of hair, is a carrier A female with a lot of hair, is a carrier for baldness has children with a man for baldness has children with a man who is losing his hair, yet is a carrier who is losing his hair, yet is a carrier (dad was not bald). (dad was not bald).

What will be the phenotypes and What will be the phenotypes and genotypes of their children?genotypes of their children?

Male Pattern BaldnessMale Pattern Baldness

XXBb x XYBb XXBb x XYBb XB Yb

XB

Xb

Genotypic Ratios:

Phenotypic Ratios:

• REMEMBER the Bb has different meanings depending on gender!!!

TYPES OF DOMINANCE:TYPES OF DOMINANCE:Exceptions to Mendel’s Law of Exceptions to Mendel’s Law of

DominanceDominance

Incomplete DominanceIncomplete Dominance

In Mendel’s law of dominance, In Mendel’s law of dominance, heterozygotes exhibited the heterozygotes exhibited the dominant phenotype. dominant phenotype.

When two alleles of a gene appear to When two alleles of a gene appear to be blended in the phenotype, the be blended in the phenotype, the alleles are said to show incomplete alleles are said to show incomplete dominance – they don’t look like dominance – they don’t look like either parent.either parent.

An example is snapdragon An example is snapdragon colourcolour

Red snapdragons Red snapdragons crossed with crossed with white snap white snap dragons yield dragons yield pink snap pink snap dragons—a dragons—a complete complete BLENDING of the BLENDING of the alleles. alleles.

In the example, In the example, note how the note how the allele is shown.allele is shown.

Another way the alleles are shown is Another way the alleles are shown is to use RR for the incompletely to use RR for the incompletely dominant red colour, and R’R’ for the dominant red colour, and R’R’ for the white colour. THUS pink flowers would white colour. THUS pink flowers would be RR’be RR’

If two pink snap dragons are crossed, If two pink snap dragons are crossed, what will be the expected phenotypes what will be the expected phenotypes and genotypes?and genotypes?

Incomplete Dominance Incomplete Dominance (cont’d)(cont’d)

__________    X   ___________________    X   _________

? ?

?

?

Genotypic Ratios:

Phenotypic Ratios:

Curly HairCurly Hair

In humans, curly hair is incompletely In humans, curly hair is incompletely dominant to straight hair. dominant to straight hair.

Children who are heterozygous will Children who are heterozygous will have wavy hair. have wavy hair.

What will be the phenotypes and What will be the phenotypes and genotypes of the children from a genotypes of the children from a female with curly hair and man with female with curly hair and man with wavy hair? wavy hair?

C represents curly hairC represents curly hair OR OR HHCC

C’ represents straight hair or C’ represents straight hair or HHSS

__________    X   ___________________    X   _________ ? ?

?

?

Genotypic Ratios:

Phenotypic Ratios:

Co-Dominance InheritanceCo-Dominance Inheritance It may seem as if incomplete It may seem as if incomplete

dominance and co-dominance are dominance and co-dominance are the same, but they are not. the same, but they are not.

When two alleles of a gene are When two alleles of a gene are clearly expressed in the phenotype, clearly expressed in the phenotype, the alleles are said to be co-the alleles are said to be co-dominant. dominant.

This results in two distinct and This results in two distinct and detectable gene products detectable gene products

Blended: At a distance, the cattle Blended: At a distance, the cattle appear roan coloured and mottled:appear roan coloured and mottled:

• The individual hairs are either red or they are white.

In doing the PUNNET square we use In doing the PUNNET square we use both capital letters for the trait—R for both capital letters for the trait—R for red and W for white.red and W for white.

Eg. A roan bull (RW) mates with a Eg. A roan bull (RW) mates with a white cow. white cow.

What will be the phenotypes and What will be the phenotypes and genotypes of their offspring?genotypes of their offspring?

Co-Dominance InheritanceCo-Dominance Inheritance (cont’d)(cont’d)

RWRW     X      X   WWWW

W W

Genotypic Ratios:

Phenotypic Ratios:

• 50% will be roan and 50% will be white

R

W

Appaloosa HorsesAppaloosa Horses

In horses, gray horses (GG) are In horses, gray horses (GG) are codominant to white horses (WW).  codominant to white horses (WW). 

The heterozygous horses(GW) is an The heterozygous horses(GW) is an appaloosa horse (a white horse with appaloosa horse (a white horse with gray spots on the rump and loins). gray spots on the rump and loins).

Cross a white horse with an Cross a white horse with an appaloosa horseappaloosa horse

__________    X   ___________________    X   _________ ? ?

?

?

Genotypic Ratios:

Phenotypic Ratios:

AssignmentAssignment

12.2 & 12.3 worksheets12.2 & 12.3 worksheets P 97, 99, 90, 91, 100, 105, 106, 92, 101P 97, 99, 90, 91, 100, 105, 106, 92, 101 98, 107-11298, 107-112

Blood TypingBlood Typing

Blood is typed according to what type(s) Blood is typed according to what type(s) of antigen (a cellular product that induces of antigen (a cellular product that induces antibody formation in a foreign host) are antibody formation in a foreign host) are found on the surface of the red blood found on the surface of the red blood cells. cells.

Blood type is determined by reacting the Blood type is determined by reacting the blood with antibody against the antigens. blood with antibody against the antigens.

Typical blood types are the ABO blood-Typical blood types are the ABO blood-groups. groups. (Text P 325 & 978)(Text P 325 & 978)

Type AB BloodType AB Blood

The AB blood type in humans is the The AB blood type in humans is the result of an individual carrying both result of an individual carrying both the the IIAA and the and the IIB B alleles.alleles.

The blood type is due to a glycoprotein The blood type is due to a glycoprotein present on the surface of red blood cells.present on the surface of red blood cells.

Blood Type and Geographic Blood Type and Geographic LocationLocation

HowStuffWorks Videos "Why Tell Me Why: Different Blood Types"

INCREASING THE INCREASING THE GENE-POOLGENE-POOL

Multiple Allelle InheritanceMultiple Allelle Inheritance

Mendel never knew that some traits Mendel never knew that some traits occur in more than pairs.occur in more than pairs.

This is called This is called multiple allele multiple allele inheritanceinheritance because more than two because more than two alleles are possible for one trait—but alleles are possible for one trait—but only two alleles are inherited and only two alleles are inherited and involved. involved.

In humans, our major blood type system In humans, our major blood type system is a classic example. is a classic example.

ABO Blood GroupingABO Blood Grouping

We commonly call it the We commonly call it the ABO ABO systemsystem. .

As a As a multiple allelemultiple allele, we write it like , we write it like we did for co-dominance—that is we did for co-dominance—that is because both the A and B are equally because both the A and B are equally strong. strong.

The exception is the type O which is The exception is the type O which is a recessive condition:a recessive condition:

IIAA - for type A antigens- for type A antigens

IIBB - for type B antigens- for type B antigens

ii - for the recessive O condition (which - for the recessive O condition (which produces neither the A nor B antigens and produces neither the A nor B antigens and does not interfere with type A or B blood).does not interfere with type A or B blood).

GenotypeGenotype Phenotype Phenotype

IIAA IIAA or or IIAA i i - type A blood (notice A - type A blood (notice A is is dominant to O) dominant to O)

IIBB IIBB or or IIBB i i - type B blood- type B blood IIAA IIBB - type AB blood (both - type AB blood (both

antigens are present) antigens are present) iiii - type O blood (no A nor - type O blood (no A nor

B B antigens are present) antigens are present)

The following allele combinationsThe following allele combinations are possible: are possible:

A woman is homozygous for type B A woman is homozygous for type B blood (Iblood (IBBIIBB)has a child with a man who )has a child with a man who is heterozygous for type A blood. is heterozygous for type A blood.

This means he is IThis means he is IAAi. i. What will be the genotypes and What will be the genotypes and

phenotypes of their children?phenotypes of their children?

IIAAii     X       X   IIBBIIBB

IA i

IB

IB

Genotypic Ratios:

Phenotypic Ratios:

What is the probability a couple What is the probability a couple whose blood types are AB and O will whose blood types are AB and O will have a child with type A blood?have a child with type A blood?

__________    X   ___________________    X   _________ ? ?

?

?

Genotypic Ratios:

Phenotypic Ratios:

POLYGENIC INHERITANCEPOLYGENIC INHERITANCE

Many characters cannot vary in a Many characters cannot vary in a population across a continuum population across a continuum (gradient). (gradient).

For example, skin color in humans is a For example, skin color in humans is a quantitative character – this means the quantitative character – this means the character is controlled by more than character is controlled by more than one geneone gene at the same time ( at the same time (polygenic polygenic inheritanceinheritance) – that is, the trait depends ) – that is, the trait depends on several chromosomal locations at the on several chromosomal locations at the same time. same time.

This is different from multiple allele This is different from multiple allele inheritance where only TWO alleles inheritance where only TWO alleles are passed on but in the population are passed on but in the population there are several types of alleles for there are several types of alleles for one trait—try not to confuse these!.one trait—try not to confuse these!.