extensions to mendel

MENDEL WAS RIGHT…. BUT NOT FOR EVERY

SITUATION…

Extensions to Mendel

Going beyond Mendel

So far you have learned about Mendel’s Dominance-Recessive Mode of Inheritance… but this is not the only method of inheritance.

Codominance Incomplete Dominance X-linked traits Multiple Alleles Polygenic Traits

Codominance

Both alleles are dominant and are fully expressed.

Examples: Coat color in cattle Human blood types Cat fur

Codominance

Example : Cattle coat color

White (WW) Roan (RW)Red (RR)

Punnett Squares with Codominance

Cross a white heifer with a red bull

a. Determine the genotypes of the parents and the gametes they will contribute to their offspring.

b. Set up your punnett square . . .

Phenotypes Genotypes Gametes

white WW W, Wred RR R, R


Cross a white heifer with a red bull

W W

R

R

WR WR

WR WR

Results:

100% WR Roan


Cross two Roans




Roan RW R, WRoan RW R, W


Cross two Roans

W

W

R

R

WR x WR

WR

WR

RR

WW

Results

25% White (WW)

50% Roan (WR)

25% Red (RR)

Both alleles contribute to the phenotype….

Incomplete Dominance

Neither allele is dominant. If both alleles are present than the phenotype will be an intermediate.

Example: snapdragons flower colors (red, white, pink) Hair texture (wavy, straight, curly)

Incomplete Dominance

Example: Snapdragons

White (rr) Pink (Rr)Red (RR)

Punnett Squares with Incomplete Dominance

Cross a White Snapdragon with a Red Snapdragon




White rr r, rRed RR R, R


R

r

Rr

Offspring:

100% Pink (Rr)

R

r Rr

RrRr


Cross Two Pink Snapdragons

a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring.



Pink Rr R, rPink Rr R, r


Cross Two Pink Snapdragons

rrr

Results

25% Red (RR)

50% Pink (Rr)

25% white (rr)RR

R

Rr

Rr

R

rRr x Rr

Example of Incomplete Dominance: Hypercholesteremia

EEee

Ee

X-linked Traits (a.k.a. sex linked)

Traits that are located on the X-chromosome

Examples:color-blindness color blind testHemophilia (blood-clotting disorder)Calico and tortoise-shell catsDuchenne’s muscular dystrophy

http://colorvisiontesting.com/

A man with hemophilia marries a woman who is homozygous normal.

In hemophilia: H - Dominant (no hemophilia)

h - recessive (hemophilia)

Punnett Squares with X-linked traits


A man with hemophilia (h) marries a woman who is homozygous normal.




Hemophiliac Man

XhY Xh, YHomozygous- normal woman

XHXH XH, XH


A man with hemophilia marries a woman who is homozygous normal.

Xh Y

XH

XH

XHXh

XHXh

XHY

XHY

Results:

Girls: (XHXh) all will be carriers with no hemophilia

Boys: (XHY) all will be normal.

Dad: XhY

Mom

: XH

XH


Lets look at color blindness – another X-linked trait.

In color blindness: B is dominant (not color blind) b is recessive (color blind)


A boy inherits his color-blindness from his mother NOT his father. Create a punnett

square to show this. . .a. Determine the genotypes of the parents and the

gametes they will contribute to their offspring.



Mom (carrier)

XBXb XB, Xb

Dad (healthy)

XBY XB, Y


So how would a boy inherit color-blindness? Create a punnett square to test your theory.

Mom

: XB

Xb

XB Y

XB

Xb

XBXB

XBXb

XBY

XbY

Results:

25% XBXB normal girl

25% XBXb carrier girl

25% XBY normal boy 25% XbY color-blind boy

Dad: XBY


Even though it is rare, girls can be color-blind if they inherit it from BOTH

parents!a. Determine the genotypes of the parents and the gametes they

wll contribute to their offspring.



Mom (carrier)

XBXb XB, Xb

Dad colorblind

XbY Xb, Y


Can girls be color-blind? How would could that happen? Create a punnet square showing your

theory.Dad: XbYXb Y

XB

Xb

XBXb

XbXb

XBY

XbY

Results:

25% XBXb carrier girl25% XbXb color-blind

girl25% XBY normal boy 25% XbY color-blind

boy

Mom

: XB

Xb

Multiple Alleles

There are more than 2 alleles, present in a certain population, for a given trait. This person will only have two alleles even though

there are more than two possibilities!Examples:

Rabbit fur color Human blood types

Punnett Squares with Multiple Alleles

Rabbit Coat Colors The brown allele (C) is dominant. The light brown allele (cch) is dominant to ch and c. The Himalayan allele (white with dark extremities)

(ch) is dominant to c. The albino allele (c) is recessive.

C > cch > ch > c

Punnett Squares with multiple alleles

What would be the result of a cross between an albino rabbit and a light brown rabbit with a albino parent?




Albino cc c, cLight Brown

cchc cch, c

Punnett Squares with Multiple Alleles

Possible Genotypes Brown (4): CC, Ccch, Cch, Cc

Light brown (3): cchcch, cchch, cchc.

Himalayan (2): chch, chc

Albino (1): cc

C > cch > ch > c


Results: 50% cchc light brown

50% cc albino

c c

cch

c

cchc cchc

cc cc

AlbinoLi

ght b

row

n


What would be the result of a light brown rabbit (with an albino mother) crossed with a heterozygous himalayan rabbit?




Light Brown

cchc cch, cHimalayan chc ch, c


Results:25% cchch light

brown

25% cchc light brown

25% chc himalayan

25% cc albino

Light brown: cchc

Himalayan: chc cch c

ch

c

cchch chc

cc

What would be the result of a light brown rabbit (with an albino mother) crossed with a himalayan rabbit?

cchc

Polygenic Traits

A trait that is controlled by two or more genes. Will manifest as a range of phenotypes.

Polygenic Traits

Examples include Eyecolor Height Skin color

Punnett Squares with polygenic traits

Even though eyecolor is controlled by at least three genes, we really understand how brown/blue/green colors work as controlled by two genes…

Eyecolor:

Gene 1: the green/blue eye color gene is located on chromosome 19.

Green is dominant (G) , blue is recessive (g)

Gene 2: the central brown eye color gene is located on chromosome 15.

Brown is dominant (B) , blue is recessive (b)


Eyecolor: Gene 1: green is dominant (G), , blue is recessive (g)

Gene 2: brown is dominant (B), blue is recessive (b) Brown eyecolor: BBGG, BBGg,

BbGg, Bbgg Green eyecolor: bbGG, bbGg Blue eyecolor: bbgg

Brown > Green > Blue


What would be the result of a cross between a blue-eyed person (recessive for both genes) and a brown-eyed person (heterozygous for both genes)



Phenotypes Genotypes

Gametes

Blue eyed

bbgg bg

Brown-eyed

BbGg BG, Bg, bG, bg


Results:bg

BG

Bg

bG

bg

BbGg

Bbgg

bbGg

bbgg

bbgg Brown > Green > Blue

BbG

g

Genotype Phenotype

25% BbGg 50% brown

25% Bbgg 25% bbGg 25% green25% bbgg 25 % blue


What is the result when a brown-eyed man (heterozygous for both genes) is crossed with a heterozygous green eyed woman?

Phenotypes Genotypes GametesGreen-eyed

bbGg bG, bg

Brown-eyed

BbGg BG, Bg, bG, bg




Results: Geno: Pheno:1/8 BbGG4/8 brown2/8 BbGg 3/8 green1/8 Bbgg 1/8 blue 1/8 bbGG2/8 bbGg1/8 bbgg

BbGg X bbGgbg

BGBg

bGbg

bG

BbGG

bbGG

bbGg

BbGg

BbGg

Bbgg

bbGg

bbgg

Polygenic trait – Skin colorHypothetically found on 3 genes: Human Genome project proposes its actually found on many more.

“Notable Notes”

Pleiotropy

Exact opposite of polygenic inheritance: A single gene affects many phenotypic characteristics

Example: Sickle-cell allele When present on

both homologous chromosomes can cause sickle-cell anemia

Heterozygotes areresistant to malaria(why allele has survived)

Pleiotropy

1 GENE

Can affect MANY phenotypes

Linked genes

• The number of genes in a cell is far greater than the number of chromosomes; in fact, each chromosome has hundreds or thousands of genes.

• Genes located close together on the same chromosome tend to be inherited together and are called linked genes.

• Linked genes generally do not follow Mendel’s law of independent assortment.

Gene mapping

Thomas Hunt MorganDrosophila fly (fruit fly) Used % outcome of

crossing over (recombination frequency) to map out where genes loci (location) are on chromosomes

Genetics and Your Environment

Many characteristics (phenotypes) result from a combination of heredity and environment.

For humans, nutrition influences height; exercise alters build; sun-tanning darkens the skin, and experience improves performance on intelligence tests.

Risk of heart disease and cancer and susceptibility to alcoholism and schizophrenia–are influenced by both genes and environment.

The End

extensions to mendel

Documents

wpunnett squares

xhxhpunnett squares

xlinked traitspunnett

wr roan punnett squares

incomplete dominancecross

white heifer

xlinked traitslets

xlinked traitsa man