Post on 24-Feb-2016
description
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
Punnett Squares with Codominance
Cross a white heifer with a red bull
W W
R
R
WR WR
WR WR
Results:
100% WR Roan
Punnett Squares with Codominance
Cross two Roans
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
Roan RW R, WRoan RW R, W
Punnett Squares with Codominance
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
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 rr r, rRed RR R, R
Punnett Squares with Incomplete Dominance
R
r
Rr
Offspring:
100% Pink (Rr)
R
r Rr
RrRr
Punnett Squares with Incomplete Dominance
Cross Two Pink Snapdragons
a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring.
b. Set up your punnett square . . .
Phenotypes Genotypes Gametes
Pink Rr R, rPink Rr R, r
Punnett Squares with Incomplete Dominance
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
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
Punnett Squares with X-linked traits
A man with hemophilia (h) marries a woman who is homozygous normal.
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
Hemophiliac Man
XhY Xh, YHomozygous- normal woman
XHXH XH, XH
Punnett Squares with X-linked traits
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
Punnett Squares with X-linked traits
Lets look at color blindness – another X-linked trait.
In color blindness: B is dominant (not color blind) b is recessive (color blind)
Punnett Squares with X-linked traits
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.
b. Set up your punnett square . . .
Phenotypes Genotypes Gametes
Mom (carrier)
XBXb XB, Xb
Dad (healthy)
XBY XB, Y
Punnett Squares with X-linked traits
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
Punnett Squares with X-linked traits
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.
b. Set up your punnett square . . .
Phenotypes Genotypes Gametes
Mom (carrier)
XBXb XB, Xb
Dad colorblind
XbY Xb, Y
Punnett Squares with X-linked traits
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?
a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring.
b. Set up your punnett square . . .
Phenotypes Genotypes Gametes
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
Punnett Squares with multiple alleles
Results: 50% cchc light brown
50% cc albino
c c
cch
c
cchc cchc
cc cc
AlbinoLi
ght b
row
n
Punnett Squares with multiple alleles
What would be the result of a light brown rabbit (with an albino mother) crossed with a heterozygous himalayan rabbit?
a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring.
b. Set up your punnett square . . .
Phenotypes Genotypes Gametes
Light Brown
cchc cch, cHimalayan chc ch, c
Punnett Squares with multiple alleles
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)
Punnett Squares with polygenic traits
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
Punnett Squares with polygenic traits
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)
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
Blue eyed
bbgg bg
Brown-eyed
BbGg BG, Bg, bG, bg
Punnett Squares with polygenic traits
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
Punnett Squares with polygenic traits
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
a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring.
b. Set up your punnett square . . .
Punnett Squares with polygenic traits
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