Genetics

Mendel’s Laws

Mendel

• Gregor Mendel was a monk.

• He grew peas in the monastery garden.

• He studied how they grew and reproduced

• He counted and calculated the outcome of many generations of pea plants

• He published his conclusions about genetics, but they were not read by other scientists for many years

Mendel’s Laws and Principles

• Genetic factors (genes) occur in pairs. One factor of each pair comes from the male parent and one from the female parent.

• Principle of Dominance and Recessiveness: The genetic factor (gene) from one parent may mask (or dominate) the genetic factor from the other parent.

• Law of Segregation: pairs of genetic factors (genes) are separated during the formation of gametes (egg or sperm)

• Law of Independent Assortment: genetic factors (genes) are distributed to gametes independently.*

* We now know that this is only strictly true when the factors are located on different chromosomes. Genes that share a chromosome may “travel together” to some extent. The “crossing-over” that occurs during the first division of meiosis does give some validity to the law of independent assortment.

What is so amazing

about Mendel is that he figured

all this out without ever seeing

a chromosome!

Quick Review of Mendel

• Your traits are controlled by genetic factors (genes for short)

• You have two of each gene (you got one of them from each parent).

• One gene sometimes overpowers your other one (dominance vs. recessiveness)

• Your genes separate (segregate) when you produce gametes (eggs or sperm)

• Your children only get half of your genes.• Its pretty much random which of your

genes they get (independent assortment)

Terminology: Allele• An allele is one of several forms of a

gene.

• Explanation: A gene can have several forms. – One form of a hair-colour gene might cause the

organism to produce a darker pigment than the other. – One form of another gene might produce more growth

hormone than its alternate.

is a GeneAn Allele

There can beseveraltypes

Everybodyhas two allelesOf each gene.

They can be thesame or different

The different types of allelecan be represented by

letters (uppercase, lowercaseor superscripted)

Examples of alleles• examples:

– Pea plants have two alleles for the height gene:• Tall height T (the dominant allele)• Dwarf height t (the recessive allele)

– Peas can get any combination of the 2 alleles– (TT)=tall plant, (Tt)=tall plant, (tt)=short plant

– Humans have 3 alleles for a blood type gene:• A type blood IA (dominant to O type)• B type blood IB (dominant to O type)• O type blood i (recessive to A and B)

– You can get any combination of 2 of these 3 alleles– (IA IA)or(IA i) = type A; (IBIB)or(IBi)=type B – (IA IB) = type AB; (i i) = type O

Genotype & Phenotype

• Phenotype: The traits that an organism actually displays (eg dark hair)

• Genotype: the underlying genes that control the trait, (usually represented by UPPER or lower case letters.)

– Eg: DD- homozygous dark hairDd- heterozygous dark hair

dd- homozygous blond hair– Homozygous: having two identical alleles of a gene.– Heterozygous: having two different alleles of a gene

(alleles are alternate forms of the gene)

Same Phenotype

Different Genotype

Generation Names• The Parental Generation (P)

– This is the two original individuals that were crossed

• The First Filial Generation (F1)

– These are the offspring (the children) of the parental generation

• The Second Filial Generation (F2)

– These are the offspring of two individuals from the F1 generation. Essentially, they are the grandchildren of the parental generation

Punnett Squares

• Developed by Reginald Punnett to help illustrate Mendel’s laws.

• A Punnett square shows the various possible outcomes of a genetic cross

Writing Genotype

• The genotype of an organism is its arrangement of its two alleles of a gene.

• If it has two alleles for tallness, its genotype will be TT, If it has one allele for tallness, and one for shortness, it will be Tt, if it has both alleles for shortness, it will be tt.

To make a Punnett Square

• Determine the genotype of the parental generation, eg Rr x Rr, or BB x bb, or YY x Yy

• Draw a square, divide it into four smaller squares.

• Above the square write the two letters corresponding the paternal genotype

• To the left, write the two letters corresponding to the maternal genotype

R r

Rr

• Go through the four boxes, picking the genotype letter from directly above, and the one from directly left, and copying them in each box

• If a genotype has an upper and lowercase of the same letter, always write the uppercase one firstrR is equivalent to Rr

Preferred way of writing it!

R r

Rr

RR

Rr

Rr

r r

• BB x bb gives the gametes:– B-B and b-b– All offspring are Bb

• Bb X Bb gives the following gametes– B-b and B-b

• The next generation:– Bb, BB, and bb

The Punnett SquareMonohybrid cross

Parental P1

First Filial F1

2nd Filial F2

Corrections for Practice SheetParental Generation: YY x yy

1st filial

YY x yyY Y 2nd filial

Yy x YyY y

y Yy Yy Y YY Yy

y Yy Yy y Yy yy

Predictions:Phenotype Yellow: 100%Phenotype green: 0%

Genotype Yy 100%Genotype YY 0%Genotype yy 0%

Predictions:Phenotype Yellow: 75%Phenotype green: 25%

Genotype Yy 50%Genotype YY 25%Genotype yy 25%

Practice sheet, Question 2

1st filial

tt x Ttt t 2nd filial

Tt xTtT t

T Tt Tt T TT Tt

t tt tt t Tt tt

Predictions:Phenotype Tall: 50%Phenotype short: 50%

Genotype Tt 50%Genotype tt 50%Genotype TT 0%

Predictions:Phenotype Tall: 75%Phenotype short: 25%

Genotype Tt 50%Genotype tt 25%Genotype TT 25%

outer

Punnett SquareDihybrid Cross

• A dihybrid cross involves two traits at the same time

• The Punnett Square gets a bit more complicated

• RrYy X RrYy– Gives the following gametes:

• RY-Ry-rY-ry (paternal)• RY-Ry-rY-ry (maternal)• How? It’s a bit like FOIL in

math. (shown for paternal gametes)

RrYyFirstLast

Inner

RYRy

rY ry

Twice as wide by twice as highThat means for each new trait the

Square is four times as complicated

the parental generation

RYRy rY ry

Since theMaternal parentHas the same

Genotype, we’llShow it quickly

Now just take an R grom the top and

an R from the sideand a Y from the topand a Y from the side

to fill up each littlebox.

The results will showall the possible

genotypes, and theirprobabilities

Genotype RatiosRRYY = 1/16= 6.25%RRYy =2/16 = 12.5%RrYY =2/16= 12.5%RRyy=1/16= 6.25%RrYy = 4/16 = 25%Rryy = 2/16 = 12.5%rrYY = 1/16 = 6.25%rrYy = 2/12 = 12.5%rryy = 1/12 = 6.25%

56.25%

18.75%

18.75%

6.25%

Phenotype Ratios

Incomplete Dominance & Co-dominance (2 types of trait blending)

• Sometimes an allele does not completely overpower the other allele.– If the traits blend, to produce an “in-between trait” we

call it incomplete dominance:• Eg. White flowers x red flowers pink flowers

– If both traits show up, we call it codominance• Eg. White hair horse x red hair horse roan

horse

black dog x white dog spotted dog

A type blood x B type blood AB type blood

The y chromosome is “stunted” or smaller than the corresponding X chromosome, so there are a few traits carried on the X chromosome that males have only one allele for.*the platypus has a slightly different chromosome system

I’m different!

Sex Determination in Mammals(except for the platypus)

• In all mammals* and many vertebrates, the sex of an organism is determined by the sex chromosomes.

• Identical sex chromosomes = (XX) female

• Different sex chromosomes = (XY) male

Sex Determination in other organisms

• Many other organisms use X and Y chromosomes to determine sex, but not all do.– Some amphibians (frogs, salamanders) can change sex

based on environmental condition, regardless of their chromosomes.

– Birds have Z and W chromosomes instead of X and Y• Females have ZW, males have ZZ, a sort of reversal of the

mammal system– Some insects have a haploid male, diploid female sex

determination, which has many strange outcomes:• Males have no fathers, and can have no sons, but they can have

grandfathers and can become grandfathers.• There are more than two sex possibilities (drone, queen, worker)

– For some species (like the zebrafish) we still don’t know how sex is determined.

Human Genetic Abnormalities(diseases and conditions due to genes)

• Diseases caused by a single gene– Huntington’s Disease is a neurological condition

caused by a rare dominant gene with 96% chance of expression. Since symptoms only develop in adults, it is easily passed on, even though it is frequently fatal.

– Cystic Fibrosis is a lung disease caused by a recessive gene

– Sickle cell anaemia is a blood condition caused by a recessive gene. It is more frequent in people whose ancestors came from tropical regions. This may be because carriers (people with one normal and one sickle cell allele) are more resistant to malaria.

• More diseases caused by a single gene– Tay-Sach’s Disease is a degeneration of the nervous

system that usually causes death in early childhood (c. Age 4). Tay-Sach’s disease is most common in people whose ancestors came from Eastern Europe, Eastern Quebec, and Louisiana.

– Phenylketonuria is the inability of the body to process the amino acid phenylalanine. It causes gradual brain damage, and if untreated, can lead to seizures and death by the end of childhood. Phenylketonuretics who receive treatment to survive childhood, must restrict their intake of proteins containing phenylalanine, and avoid the sweetener aspartame (AKA. NutraSweet) which also contains phenylalanine.

• X-linked (AKA. Sex-linked) Conditions.

• These are usually prevented by genes carried on the X chromosome, so they are more frequent in males than females.– Haemophilia is a condition that prevents the

blood from clotting normally. A victim can die of minor cuts unless treated quickly.

– Red/Green Colourblindness is the most common form of colour deficiency in human males.

– Muscular Dystrophy is a disease of the muscles fibres that causes weakness and reduced life expectancy

Colourblindness test

25(everyone)

45

6

29

56

8

How X-Linked Traits Work• Most X-linked traits are caused by a recessive allele located

on the X-Chromosome. Because Y chromosomes do not have all the genes found on an X chromosome, men are less likely to have the dominant normal gene that would prevent the condition. Therefore, X-linked traits are several times more likely to be expressed in a male than in a female

• Two well known sex-linked traits are R/G colourblindness and hemophilia– XC = gene for normal vision XCXC= normal

female– Xc = gene for colourblindness XCXc= female carrier (normal)

– Y = no gene XcXc= colourblind female

XCY= normal maleXcY =colourblind

male

Let’s say 90% of all genes are normal and 10% are not. That means a female has a 99% chance of normal vision, but a malehas only a 90% chance of normal vision, sothe male is about 10 times more likely to beColourblind.

81%18%

1%90%10%

Missing Chromosome (monosomy) and

Extra Chromosome (trisomy) Diseases• Down Syndrome (trisomy-21) results from

having an extra copy of chromosome 21. • Symptoms include impaired mental development,

characteristic facial features, and occasionally muscle weakness.

• Klinefelter’s Syndrome (trisomy-XXY)• Affects a small percentage of men. Symptoms

include small testicles, reduced male hormone production and higher incidence of male cancers and some other diseases, such as diabetes and rheumatoid arthritis.

• Turner’s Syndrome (monosomy-XO)• About 1 in 5000 females has a missing sex

chromosome. This can result in several characteristics, including:

• Short stature, low-set ears, low hairline, webbed neck, amenorrhea, and sterility.

• Trisomy-X (trisomy-XXX) • An extra X chromosome produces a female that is

normal to most external appearances, apart from a tendency to be taller than average. There is some risk of increased minor disorders, including:

– behavioural issues, clumsiness, poor coordination, underdeveloped facial muscles, wide-set eyes, etc.

Other chromosome related conditions• Cri-du-chat syndrome

– Characterized by delayed development, small head size, and characteristic facial features. Infants often have high pitched cry.

– Caused by a loss of part or all of chromosome #5

• Angelman Syndrome– Characterized by intellectual difficulties, seizures, and small head

size.– Caused by malfunction of a gene on chromosome #15 or by a

missing maternal chromosme#15 – This is one of the few cases where it seems to matter who you

inherited the chromosome from. The maternal chromosome always seems to dominate.

– Children with Angelman syndrome typically have a happy, excitable demeanor with frequent smiling, laughter, and hand-flapping movements. Hyperactivity and a short attention span are common. Most affected children also have difficulty sleeping and need less sleep than usual. Some affected individuals have unusually fair skin and light-colored hair

• XYY Syndrome (trisomy-XYY, Jacob’s Syndrome)

• An extra Y chromosome produces a male that is in most respects normal. He may have slight symptoms of excess male hormones, such as being slightly taller than average, and having a tendency towards acne.

• At one time it was believed that men with this condition had a higher tendency towards violent crime, but this hypothesis has not been born out by further studies. It may affect as many as 0.1% of the male population.

In Summary

• Phenotype is the actual appearance of a certain trait (eg: dark hair, blond hair)

• Genotype is the underlying pair of genes controlling the trait (eg: BB, Bb or bb)

• Alleles are alternate forms of a gene, that occur in “pairs of genes”

• Homozygous for a trait means having the same alleles (eg: BB or bb) of the gene of the trait.

• Heterozygous means having different alleles (eg: Bb)

A pair of genes

• All genes (except a few on the Y chromosome) occur in pairs - two alleles for each trait.

• One allele may overpower another (ie. It is dominant)

• If you are heterozygous for a trait, the dominant allele wins!

Assignments

• Read p. 165-178

• Read the vocabulary lists on page 179. look up any terms that you don’t recognize.

• Do the exercises on page 180 to 181 questions # 6-25 (except #18 in the green book). Put the answers on paper, to be handed in next class. Write the question as well as the answer.