Chapter 16fd.valenciacollege.edu/file/rluther1/ch16_lecture_ppt_rvsd.pdf2 Chapter 16 Simple Patterns of Inheritance Mendel’s Laws of Inheritance Chromosome Theory of Inheritance

Chapter 16

Lecture Outline

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Chapter 16 Simple Patterns of Inheritance

  Mendel’s Laws of Inheritance

  Chromosome Theory of Inheritance

  Pedigree Analysis of Human Traits

  Sex Chromosomes and X-linked Inheritance

  Molecular Basis of Different Inheritance Patterns

  Genetics and Probability

Key Concepts:

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Mendel’s Laws of Inheritance

  Gregor Mendel, 1822-1884

  Entered monastery and became a priest

  Historic experiments with pea plants

  His paper was ignored at the time, but his findings were independently rediscovered years later

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Garden Pea, Pisum sativum

  Many different variable traits

  Normally self-fertilizing   Female gamete fertilized by male gamete from same plant

  Easy to breed true-breeding lines (exhibit the same trait)

  Large flowers make crosses easy when desired   Cross-fertilization or hybridization

Several advantageous properties:

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Character

Flower color

Purple White

Variants (Traits)

Flower position

Axial Terminal

Seed color

Yellow Green

Seed shape

Round Wrinkled

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Character Variants (Traits)

Pod color

Green Yellow

Pod shape

Smooth Constricted

Height

Tall Dwarf

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Stigma

Stamen

Ovary

Ovule


© Nigel Cattlin/Photo Researchers, Inc.

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Transfer pollen from stamens of white flower to the stigma of a purple flower.

Remove stamens from purple flower.

1 2

Stamens

Stigma


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  P generation   True-breeding parents

  F1 generation   Offspring of P cross

  Monohybrids (if parents differ in one trait)

  F2 generation   F1 self-fertilizes

  Recessive trait reappears

Single-factor cross Where the experimenter follows only a single trait


×

All tall offspring (monohybrids)

Experimental approach

P generation

Tall Dwarf

Cross-fertilization

F1 generation

F2 generation

Tall offspring Dwarf offspring

(a) Mendel’s protocol for making monohybrid crosses

3 1 :

Inheritance pattern

TT × tt

All Tt (tall)

TT tt Tt

(Tall) (Dwarf)

1 : 2 : 1

Self-fertilization

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(b) Mendel’s observed data for all 7 traits

Round wrinkled seeds

Yellow green seeds

Purple white flowers

Axial terminal flowers

Tall dwarf stem

F2 generation

5,474 round, 1,850 wrinkled

6,022 yellow, 2,001 green

705 purple, 224 white

651 axial, 207 terminal

787 tall, 277 dwarf

14,949 dominant, 5,010 recessive

Smooth constricted pods

882 smooth 299 constricted

Green yellow pods

428 green, 152 yellow

THEDATA

P cross

Total

F1 generation

All purple

All axial

All yellow

All round

All green

All smooth

All tall

All dominant

Ratio

3.15:1

3.14:1

3.01:1

2.96:1

2.82:1

2.95:1

2.84:1

2.98:1

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Mendel’s three important ideas

1.  Traits are dominant and recessive   Dominant variant is displayed in hybrids   Recessive variant is masked by dominant

2.  Genes and alleles   Particulate mechanism of inheritance   His “unit factors” are genes   Every individual has two genes for a character   A gene has two variant forms, or alleles

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3.  Segregation of alleles   Two copies of a gene carried by an F1 plant

segregate (separate) from each other, so that each sperm or egg carries only one allele

  F2 traits follow approximately 3:1 ratio

Mendel’s Law of Segregation

Two copies of a gene segregate from each other during the transmission from parent to offspring.

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Tt

TT Tt Tt tt

T × t

Tt

T × t

Gametes

F1 generation

3 tall offspring

1 dwarf offspring

F2 generation

Segregation: Alleles separate into different haploid cells that eventually give rise to gametes.

Fertilization: During fertilization, male and female gametes randomly combine with each other.

1

2

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Genotype and phenotype

Genotype – The genetic composition of an individual

 TT – homozygous dominant

  tt – homozygous recessive

 Tt – heterozygous

Phenotype – Physical or behavioral characteristics that are the result of gene expression

 TT and Tt are tall

  tt is dwarf

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Punnett square

Step 1. Write down genotypes of parents   Male parent: Tt

  Female parent: Tt

Step 2. Write down the possible gametes that each parent can make

  Male gametes: T or t

  Female gametes: T or t

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Step 3. Create an empty Punnett square.

T t

♀

♂ Male gametes

T

t

Fem

ale

gam

etes

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T

T

t

t

♀

♂ Male gametes

Fem

ale

gam

etes

TT Tt

Tt tt

Step 4. Fill in the possible genotypes.

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Step 5. Determine relative proportions of genotypes and phenotypes.

T t

♀

♂ Male gametes

TT Tt

Tt tt

Genotype ratio

TT:Tt:tt

1:2:1

Phenotype ratio

tall:dwarf

3:1

T

t

Fem

ale

gam

etes

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Testcross

  A dwarf pea plant must be tt

  A tall pea plant could be either TT or Tt, so genotype must be determined by a testcross

  Cross the unknown individual (TT or Tt) to a homozygous recessive individual (tt)   If some offspring are dwarf, unknown individual must

have been Tt   If all offspring are tall, unknown individual was TT

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Dominant phenotype; genotype could be TT or Tt

Recessive phenotype; genotype must be tt

If plant with dominant phenotype is TT, all offspring will be tall.

Alternatively, if plant with dominant phenotype is Tt, half of the offspring will be tall and half will be dwarf.

T T

t

t

Tt Tt

Tt Tt

T t

t

t

×

tt

tt Tt

Tt

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Two-factor cross

  Follows inheritance of two different traits

  Can determine linkage

  Possible patterns:  Two genes are linked – variants found together

in parents are always inherited as a unit

 Two genes are independent – variants are randomly distributed

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YYRR yyrr

×

YYRR yyrr

×

YyRr YyRr

P generation

Gametes

F1 generation

Sperm Sperm

Eg

g

Eg

g

YYRR

YYRr

YyRR

YyRr Yyrr yyRr yyrr

yyRr yyRR

YyRr yyrr

YyRr YYRR

YyRr

YYrr

YYRr YyRR YyRr

Yyrr YyRr

F2 generation

(a) Hypothesis: linked assortment

(b) Hypothesis: independent assortment

yr YR

YR yr

YR

yr

YR yr

YR yR yr Yr

YR

yr

Yr

yR

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  Dihybrid offspring – offspring are hybrids with respect to both traits

  Data for F2 hybrids is consistent with independent assortment

P cross 315 yellow, round seeds 101 yellow, wrinkled seeds 108 green, round seeds

32 green, wrinkled seeds

(c) The data observed by Mendel

×

F1 generation

F2 generation

Yellow, round seeds

Yellow, round seeds

Green, wrinkled seeds



Sperm Sperm

Eg

g

Eg

g

YYRR

YYRr

YyRR

YyRr Yyrr yyRr yyrr

yyRr yyRR

YyRr yyrr

YyRr YYRR

YyRr

YYrr

YYRr YyRR YyRr

Yyrr YyRr

F2 generation YR yr

YR

yr

YR yR yr Yr

YR

yr

Yr

yR

Round

s

S

S s

y

Y

Y y

SS S

S s s s

s YY Y

Y y y y

y

Yellow

Dihybrid Cross

Probabilities Round=SS, Ss is ¾ Wrinkled=ss is ¼

Probabilities Yellow=YY, Yy is ¾ Green=yy is ¼

  Yellow and round  Proportion of peas that are yellow = ¾

 Proportion of peas that are round = ¾

 To determine the proportion of both yellow and round you have to multiply the proportion of each individual phenotype, thus,

 ¾ x ¾ = 9/16 are yellow and round

Calculating Genetic Probabilities with Mendelian Inheritance

  Yellow and wrinkled  Proportion of peas that are yellow = ¾

 Proportion of peas that are wrinkled = ¼

 To determine the proportion of both yellow and wrinkled you have to multiply the proportion of each individual phenotype, thus,

 ¾ x ¼ = 3/16 are yellow and wrinkled



  Green and round  Proportion of peas that are green = ¼

 Proportion of peas that are round = ¾

 To determine the proportion of both green and round you have to multiply the proportion of each individual phenotype, thus,

 ¼ x ¾ = 3/16 are green and round


  Green and wrinkled  Proportion of peas that are green = ¼

 Proportion of peas that are wrinkled = ¼

 To determine the proportion of both green and wrinkled you have to multiply the proportion of each individual phenotype, thus,

 ¼ x ¼ = 1/16 are green and wrinkled

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Law of Independent Assortment

Alleles of different genes assort independently of each other during gamete formation.

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Chromosome Theory of Inheritance

1.  Chromosomes contain the genetic material (DNA). Genes are found in the chromosomes.

2.  Chromosomes are replicated and passed from parent to offspring. They are also passed from cell to cell during the development of a multicellular organism.

3.  The nucleus of a diploid cell contains two sets of chromosomes, found in homologous pairs. Maternal and paternal sets of homologous chromosomes are functionally equivalent; each set carries a full complement of genes.

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Chromosome Theory of Inheritance

4.  At meiosis, one member of each chromosome pair segregates into each daughter nucleus. During the formation of haploid cells, the members of different chromosome pairs segregate independently of each other.

5.  Gametes are haploid cells that combine to form a diploid cell during fertilization, with each gamete transmitting one set of chromosomes to the offspring.

Chromosomes and segregation

  Mendel’s Law of Segregation can be explained by the pairing and segregation of homologous chromosomes during meiosis

  The physical location of a gene on a chromosome is called its locus


Gene locus—site on chromosome where a gene is found. A gene can exist as 2 or more different alleles.

T—Tall allele

t—Dwar fallele Genotype: Tt (heterozygous)

Pair of homologous chromosomes

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Homologs segregate into separate cells during meiosis I.

2

Chromosomes replicate, and cell progresses to metaphase of meiosis I.

Sister chromatids separate during meiosis II to produce 4 haploid cells.

1

3

Diploid cell

Four haploid cells

Heterozygous (Tt) cell from a tall plant

Homologues paired with each other

t t T T

t t T T

t t T T

t T

Sister chromatids

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Chromosomes and independent assortment

  The Law of Independent Assortment can also be explained by the behavior of chromosomes during meiosis

  Random alignment of chromosome pairs during meiosis I leads to the independent assortment of genes found on different chromosomes

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Four haploid cells Four haploid cells

y Y

R r

y Y

R r

y y Y Y Y Y y y

r r R R

y

R R

Y Y Y

Y Y

or

Heterozygous diploid cell (YyRr) to undergo meiosis

Heterozygous diploid cell (YyRr) to undergo meiosis

R R

y Y

r r

y y

r r

R R y

r y

r

r r R R

y R

y r r

Y Y R

Metaphase I (can occur in different ways)

Chromosomes replicate, and cell progresses to metaphase of meiosis I. Alignment of homologs can occur in more than one way.

1

Homologs segregate into separate cells during meiosis I.

2

Sister chromatids separate during meiosis II to produce 4 haploid cells.

3

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Pedigree Analysis of Human Traits

  Inherited trait is analyzed over the course of several generations in one family

  Cystic fibrosis (CF) example

 Approximately 3% of Americans of European descent are heterozygous carriers of the recessive CF allele, and phenotypically normal

  Individuals who are homozygous exhibit disease symptoms

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A family pedigree for Cystic Fibrosis, a recessive trait.

Unaffected individual

Affected individual

Presumed heterozygote

Female

Male


(a) Human pedigree showing cystic fibrosis

I

II

III

I-1 I-2

II-1 II-2 II-3 II-4 II-5

III-1 III-2 III-3 III-4 III-5 III-6 III-7

Recessive Inheritance

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  Many of the alleles causing human genetic disease are recessive, like Cystic Fibrosis

  But some are dominant, like Huntington disease   Huntington disease has an autosomal dominant

inheritance pattern   Gene is on one of 22 pairs of autosomes

  Disease genes can also be found on the sex chromosomes

Disease genes can be recessive or dominant, autosomal or sex-linked

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A family pedigree for Huntington disease, an autosomal dominant trait.

I

II

III

II-1 II-2 II-3

I-1 I-2

II-4 II-5 II-6 II-7

III-1 III-2 III-3 III-4


Dominant Inheritance

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Molecular Basis of Different Inheritance Patterns

Simple Mendelian inheritance  Alleles are dominant or recessive  Phenotype ratios follow Mendel’s laws

More complex forms of inheritance   Incomplete dominance  Codominance

Understanding gene function at the molecular level explains differences in inheritance patterns

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  Recessive allele does not affect phenotype of heterozygote

  Single copy of the dominant allele makes enough functional protein to provide a normal phenotype, masking recessive allele

  Sometimes heterozygote may even upregulate the lone functional allele to provide high enough expression

Simple Mendelian Inheritance

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Example: Purple pigment, P

  One P allele makes enough functional protein to provide a normal phenotype


Genotype

Amount of functional protein P produced

Phenotype

The relationship of the normal (dominant) and mutant (recessive) alleles displays simple Mendelian inheritance.

Colorless precursor molecule

Protein P Purple pigment

100% 50% 0%

Purple Purple White

PP Pp pp

Recessive Alleles That Cause Diseases May Have Multiple Effects on Phenotype

  In many human genetic diseases, a recessive allele fails to produce a specific functional protein

  Over 7,000 human disorders are caused by mutations in single genes

  Most single-gene diseases are recessive, but some are dominant

  Pleiotropy – mutation in a single gene has multiple effects

Example: Cystic Fibrosis (CF)

  Normal CF allele codes for transporter protein that regulates chloride ion balance

  Mutation diminishes function of transporter, causing multiple pleiotropic effects:

 Thick mucus in lungs is due to water imbalance caused by ion balance

 Sweat is very salty because salt cannot be recycled back into body without transporter

 Some males are infertile because Cl- transporter is needed for proper development of vas deferens

Incomplete dominance

  Heterozygote shows intermediate phenotype

  Neither allele is dominant

  example: Pink four-o’clocks   50% of normal protein

not enough to give red color


CRCR

CRCw

CR

CR

CR

Cw

Cw

Cw

F2 generation

Eg

g

CRCw CwCw

CRCw CRCR

P generation Red White

Gametes

CwCw

×

Self-fertilization of F1 off spring

F1 generation

Sperm

Pink

  Multiple alleles – three or more variants in a population

  Phenotype depends on which two alleles are inherited

  example: ABO blood types in humans  Type AB is codominant – expresses both alleles equally

Codominance

O A B A B

Table 16.3 The ABO Blood Group

Antigen A Antigen B

Galactose RBC

Antigen A Antigen B

RBC

AB

IAIB IBIB or IBi

Against A

A and B

Neither

RBC N-Acetyl- galactosamine

IAIA or IAi

Against B

Neither A nor B

Against A and B

ii

RBC

Blood type:

Genotype:

Surface antigen:

Antibodies:


Figure 12.13 Codominance: ABO Blood Reactions Are Important in Transfusions

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Role of environment

  The environment plays a vital role in phenotype

  Genotype provides the plan to create a phenotype; the environment provides nutrients and energy to carry out the plan

  Norm of reaction – effects of environmental variation on a phenotype

  example: Genetically identical plants grow to different heights in different temperatures

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1

2

0

3

45 55 65 75 85 95

Temperature (°F)

Hei

gh

t (f

eet)


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Role of environment

Example: Phenylketonuria (PKU) disease

 Can develop normally if given a diet free of phenylalanine

  If diet contains phenylalanine, symptoms include mental retardation, underdeveloped teeth and foul-smelling urine

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Environment influences expression of PKU within a family.The child in the middle was raised on a phenylalanine-free diet; the ones on either side were not.

Figure 12.11 Multiple Alleles Generate Diversity: Rabbit Coat Color

C: Dark gray cch: Chinchilla ch: Light gray c: Albino

Hierarchy of Dominance

Figure 12.14 Genes May Interact Epistatically (Part 1)

Gene 1 B or b: Which pigment (B is dominant to b) Gene 2 E or e: Determines if there is pigment (E is dominant to e)

Epistasis: Phenotype of one gene is affected by another gene

Figure 12.14 Genes May Interact Epistatically (Part 2)

Genetic Linkage and Recombination   Recombination frequency (RF):

 Two loci are closer, there is less crossing over

 Higher RF means two genes are farther away

 Low RF means two genes are very close and are less likely to be separated by recombination

Figure 12.21 Steps toward a Genetic Map Arbitrary reference point

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Genetics and Probability

  Probability – the chance that an event will have a particular outcome

  For a single coin toss, chance of getting heads

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Example: Self-fertilization of a pea plant heterozygous for the height gene (Tt)

 Punnett square predicts that 1/4 of the offspring will be dwarf

Tt

Tt TT

tt

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Sample size

  Prediction accuracy depends on number of events observed – the sample size

  Random sampling error – deviation between observed and expected outcome

  Larger samples have smaller sampling errors

  Humans have small families and observed data may be very different from expected outcome

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Product rule

  Probability that two or more independent events will occur is equal to the product of their individual probabilities

  If we toss a coin twice, what is the probability that we will get heads both times?

The product rule says that it is equal to the probability of getting heads on the first toss (1/2) times the probability of getting heads on the second toss (1/2), or one in four

½ x ½ = ¼

Sum rule

  Probability that one of two or more mutually exclusive outcomes will occur is the sum of the probabilities of the possible outcomes

  In a cross between two heterozygous (Tt) pea plants, we may want to know the probability of a particular offspring being a homozygote (either TT or tt)

¼ + ¼ = ½ Half the offspring will be homozygotes

Tt

Tt TT

tt

Documents

Chapter 16fd.valenciacollege.edu/file/rluther1/ch16_lecture_ppt_rvsd.pdf2 Chapter 16 Simple Patterns of Inheritance Mendel’s Laws of Inheritance Chromosome Theory of Inheritance