Genetics #2 Genetics can be very complicated. Polyallelic Traits When a trait is caused by more than two alleles in a population. An individual still only inherits two alleles for the trait one from each parent. However, they may be two entirely different alleles from its neighbor. For example: Coat color in Rabbits There are four alleles contributing to coat color: C, c, c ch, and c h Full color: CC, Cc ch, Cc h, Cc Chinchilla: c ch c h, c ch c ch, c ch c Himalayan: c h c, c h c h Albino: cc
Traits produced by the inheritance of several genes in an individual. There is an additive effect of two or more genes on a single phenotype, which results in a wide range of phenotypes along a continuum. For example, there is evidence that skin pigmentation in humans is controlled by at least 3 separately inherited genes (probably more, but we will simplify). There is a dark-skin allele for each gene (A, B, C) contributing one unit of darkness to the phenotype and becoming incompletely dominant to the other alleles (a, b, c). An AABBCC person would be very dark, while an aabbcc person would be very light. What is another polygenic trait? Polygenic Traits Can one gene stop another? Epistasis = from the Greek for stopping A gene at one locus alters the phenotypic expression of a gene at a second locus. In mice, black coat color (B) is dominant to brown (b). A second gene determines whether or not pigment will be deposited in the hair. The dominant allele (C) results in the deposition of either black or brown pigment, depending on the genotype at the first locus. But if the mouse is homozygous recessive for pigment (cc), then the coat is white (albino), regardless of the genotype at the black/brown locus. The gene for pigment deposition is said to be epistatic to the gene that codes for black or brown pigment.
Can the environment have an effect on phenotype? YES! For example, a single tree, locked into its inherited genotype, has leaves that vary in size, shape, and greenness, depending on exposure to wind and sun. The outcome of a genotype lies within its norm of reaction, a phenotypic range that depends on the environment in which the genotype is expressed. Hydrangea flowers of the same genetic variety range in color from blue-violet to pink, depending on the acidity of the soil. Can you think of environmental influences that affect human traits? The ABO Blood groups in humans is an example of a polyallelic trait showing both complete dominance and codominance. There are three alleles that contribute to human blood type: I A, I B, and i. There are four phenotypes for blood type: A, B, AB, and O. Blood Type Genotype Phenotype 1. type A = I A I A or I A i A 2. type B = I B I B or I B i B 3. type AB = I A I B AB 4. type O = ii O
What type of blood can a person with type A receive in a blood transfusion? A or O Human Blood Types What type of blood can a person with type AB receive in a blood transfusion? A, B, AB, or O Type O is called the Universal Donor. Type AB is called the Universal Recipient. ANSWER #1: Blood Type A homozygous male with type B blood mates with a heterozygous female with type A blood. What is the chance that their child will be type B blood? STEP #1: Create a Key. STEP #2: Determine Parent s Genotype P: I B I B x I A i (male) (female) I B I A I A I B i I B i STEP #3: Fill in the Punnett Square I B I A I B I B i STEP #4: Determine the phenotypes of F 1 offspring. STEP #5: Answer the question. There is a 50% that their child will have type B blood. Phenotypic Ratio: 0 Type A ½ Type B ½ Type AB 0 Type O
ANSWER #2: Blood Type If a boy has a blood type O and his sister has blood type AB, then what are the genotypes and phenotypes of their parents? Boy: type O (ii) Girl: type AB (I A I B ) Parent s must be: type A (I A i) and type B (I B i) I B i I A I A I B i ii Sex-linked Traits Traits (genes) that are located on the sex chromosomes. fruit fly eye color XX chromosome - female Xy chromosome - male Why are 99% of sex-linked genes found on the X chromosome rather than the Y chromosome? Because the X is larger than the Y. Since sex-linked genes are mostly on the X chromosome, then sons inherit these genes from which parent mother or father? The mother gives the X to the son; the father gives the Y to the son.
Female Carriers In a sex-linked trait (such as hemophilia),only females are carriers, because they have two X chromosomes therefore allowing them to carry two versions of the gene. Since males only have one X chromosome, they either express the trait or not. Therefore, sex-linked traits are more commonly expressed in males. PRACTICE PROBLEM #1: Sex-linked In fruit flies, eye color is sex-linked. Red eyes are dominant to white eyes. If a red-eyed male is mated with a white-eyed female, then what percentage of the F 1 offspring will have white eyes? STEP #1: Make a Key. R red eye color ; r white eye color XX female ; XY male STEP #2: Determine the Parent s Genotypes. P: x (male) (female) STEP #3: Fill in the Punnett Square STEP #4: Determine the Phenotypic Ratio of the F 1 offspring. STEP #5: Answer the question. Phenotypic Ratio: red-eyed male white-eyed male red-eyed female white-eyed female
ANSWER #1: Sex-linked In fruit flies, eye color is sex-linked. Red eyes are dominant to white eyes. If a red-eyed male is mated with a white-eyed female, then what percentage of the F 1 offspring will have white eyes? STEP #1: Make a Key. R red eye color ; r white eye color XX female ; XY male X R X r X R X r X r X R X r STEP #2: Determine the Parent s Genotypes. P: X R Y x X r X r (male) (female) Y X r Y X r Y STEP #3: Fill in the Punnett Square STEP #4: Determine the Phenotypic Ratio of the F 1 offspring. STEP #5: Answer the question. There is a 50% chance of a F 1 offspring having white eyes. Phenotypic Ratio: 0 red-eyed male ½ white-eyed male ½ red-eyed female 0 white-eyed female ANSWER #2: Sex-linked Hemophilia in humans is due to an X-chromosome mutation. It is a recessive trait. What will be the results of a mating between a normal (non-carrier) female and a hemophiliac male? Use the letter N to represent the non-hemophiliac and n to represent the hemophiliac. KEY: X N normal ; X n hemophilia XX female ; XY male X N X N P: X N X N x X n Y X n X N X n X N X n Phenotypic Ratio of F 1 offspring: 100% normal offspring (The females are carriers.) Y X N Y X N Y
X-inactivation in Female Mammals Female mammals inherit two X chromosomes, but one X in each cell becomes almost completely inactivated during embryonic development. Females consist of a mosaic of two types of cells, because X inactivation occurs randomly. The inactive X in each cell condenses into a compact object called a Barr body. Most of the genes of the Barr body are not expressed. In the ovaries, Barr body chromosomes are reactivated in the cells that give rise to eggs, so every female gamete has an active X.