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... PhenotypesGenotypesGametes whiteWWW, W redRRR, R

Punnett Squares with Codominance Cross a white heifer with a red bull WW R R WRWRWRWR WRWRWRWR 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... PhenotypesGenotypesGametes RoanRWR, W RoanRWR, W

Punnett Squares with Codominance Cross two Roans W W R R WR x WR WRWR WRWRR 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... PhenotypesGenotypesGametes Whiterrr, r RedRRR, R

Punnett Squares with Incomplete Dominance R r Rr Offspring: 100% Pink (Rr) R r Rr

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... PhenotypesGenotypesGametes PinkRrR, r PinkRrR, r

Punnett Squares with Incomplete Dominance Cross Two Pink Snapdragons rr r Results 25% Red (RR) 50% Pink (Rr) 25% white (rr) RR R Rr R r Rr x Rr

Example of Incomplete Dominance: Hypercholesteremia

EE ee Ee

X-linked Traits ( a.k.a. sex linked) Traits that are located on the X-chromosome Examples:  color-blindness color blind testcolor blind test  Hemophilia (blood-clotting disorder)  Calico and tortoise-shell cats  Duchenne’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

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... PhenotypesGenotypesGametes Hemophiliac Man XhYXhY X h, Y Homozygous- normal woman XHXHXHXH X H, X H

Punnett Squares with X-linked traits A man with hemophilia marries a woman who is homozygous normal. XhXh Y XHXH XHXH XHXhXHXh XHXhXHXh XHYXHY XHYXHY Results: Girls: (X H X h ) all will be carriers with no hemophilia Boys: (X H Y) all will be normal. Dad : X h Y Mom : X H X H

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... PhenotypesGenotypesGametes Mom (carrier) XBXbXBXb X B, X b Dad (healthy) XBYXBYX B, Y

Punnett Squares with X-linked traits So how would a boy inherit color-blindness? Create a punnett square to test your theory. Mom : X B X b XBXB Y XBXB XbXb XBXBXBXB XBXbXBXb XBYXBY XbYXbY Results: 25% X B X B normal girl 25% X B X b carrier girl 25% X B Ynormal boy 25% X b Y color-blind boy Dad: X B Y

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... PhenotypesGenotypesGametes Mom (carrier) XBXbXBXb X B, X b Dad colorblind XbYXbYX b, 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: X b Y XbXb Y XBXB XbXb XBXbXBXb XbXbXbXb XBYXBY XbYXbY Results: 25% X B X b carrier girl 25% X b X b color-blind girl 25% X B Y normal boy 25% X b Y color-blind boy Mom : X B X b

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 (c ch ) is dominant to c h and c.  The Himalayan allele (white with dark extremities) (c h ) is dominant to c.  The albino allele (c) is recessive. C > c ch > c h > 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... PhenotypesGenotypesGametes Albino cc c, c Light Brown c ch cc ch, c

Punnett Squares with Multiple Alleles Possible Genotypes  Brown (4): CC, Cc ch, Cc h, Cc  Light brown (3): c ch c ch, c ch c h, c ch c.  Himalayan (2): c h c h, c h c  Albino (1): cc C > c ch > c h > c

Punnett Squares with multiple alleles Results: 50% c ch c light brown 50% cc albino c c c ch c c ch c cc Albino Light brown

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... PhenotypesGenotypesGametes Light Brown c ch cc ch, c Himalayan chcchc c h, c

Punnett Squares with multiple alleles Results: 25% c ch c h light brown 25% c ch c light brown 25% c h c himalayan 25% cc albino Light brown: c ch c Himalayan: c h c c ch c chch c c ch c h chcchc cc What would be the result of a light brown rabbit (with an albino mother) crossed with a himalayan rabbit? c ch c

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... PhenotypesGenotypesGametes Blue eyedbbggbg Brown-eyed BbGgBG, Bg, bG, bg

Punnett Squares with polygenic traits Results: bg BG Bg bG bg BbGg Bbgg bbGg bbgg Brown > Green > Blue BbGg GenotypePhenotype 25% BbGg50% brown 25% Bbgg 25% bbGg25% green 25% bbgg25 % 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? PhenotypesGenotypesGametes Green-eyed bbGgbG, bg Brown-eyed BbGgBG, 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 brown 2/8 BbGg3/8 green 1/8 Bbgg1/8 blue 1/8 bbGG 2/8 bbGg 1/8 bbgg BbGg X bbGg bg BG Bg bG bg bG BbGG bbGG bbGg BbGg Bbgg bbGg bbgg

Polygenic trait – Skin color Hypothetically 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 are resistant to malaria (why allele has survived)

PleiotropyPleiotropy 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 Morgan Drosophila 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