Genetics Beyond Mendel Incomplete Dominance Neither allele is

Genetics: Beyond Mendel

Incomplete Dominance: Neither allele is dominant, “blending” of both alleles to create a different phenotype. Example: If snapdragons have a red flower allele and a white allele, flowers are pink.

Representing Incomplete Dominance Since neither allele is dominant, alleles with incomplete dominance are represented as a superscript above a letter representing the gene. Example: Red color – CR White color - CW

Codominance Both alleles are expressed in a heterozygote. Example: Roan cattle with mixed red and white hairs are heterozygotes for red and white hair color alleles.

Representing Codominance As with incomplete dominance, codominance is represented with a superscript. Example: red CR, white CW The difference between incomplete dominance and codominance is subtle, and they can be treated the same.

Applying the Concepts Which produces the greatest number of roan (CRCW) offspring? CR Roan x Roan (CRCW x CRCW) CW CR CRCW CW OR CRCW CWCW CR CR CW CRCw CRCW Red x White (CR CR x CWCW) ALL ROAN CW CRCW

For incomplete dominance, the phenotype ratio is the same as the genotype ratio. A heterozygote has a different phenotype than either homozygote in incomplete dominance, so there are three possible phenotypes.

Multiple Alleles For some traits, more than two possible alleles exist. Example: 3 Blood Type Alleles A ( I A) B (IB) O (i) A and B are codominant O is recessive

Blood type AB has both marker proteins (A and B) on red blood cell surface.

Blood Type (Phenotype) A B AB O Possible Genotype(s) IAIA or IAi IBIB or IBi IAIB only ii only

Paternity Tests A mother has blood type A, and her child has blood type O. Can a man with blood type B be the father? Yes, the mother can be IAi and the father can be IBi. A mother has blood type B and her child has blood type O. Can a man with blood type AB be the father? No, the child must have received an i allele from each parent.

Gender Determination In humans, the 23 rd pair of chromosome (X/Y) are the sex chromosomes which determine the gender. Men are XY (X and Y are the only nonhomologous pairwhich means that the pair does not contain the same type of gene. ) Woman are XX The other 22 pairs of chromosomes are called autosomal chromosomes.

MALE FEMALE

Autosomal Chromosomes Sex Chromosomes

Sex Linked Traits for which the gene is found on the X chromosome. Since men are XY, they have only one copy of sex-linked genes. Examples: Red-Green Colorblindness Hemophilia

Colorblindness Test

Sex-Linked Traits Video Click once on image to start video https: //www. youtube. com/watch? v=H 1 Ha. R 47 Dqfw

http: //www. accessexcellence. org/HHQ/qow/qo w 09/qow 090810. php

Applying the Concepts Colorblindness is caused by a recessive allele on the X chromosome (Xb). If a colorblind woman(Xb. Xb) marries a man with normal vision (XBY), what is the probability of a daughter being colorblind? A son being b b X X colorblind? X b x X B Y XB XB Xb Y X b. Y Daughter being CB: O% Son being CB: 100%

Wording of Probability in Sex-Linked Trait Questions If the question asks: “What is the probability of the parents having a daughter that has normal color vision”- it is not a given that the child is a daughter, so divide by all 4 possibilities = 2 out of 4 = 50% If the question asks: “What is the probability of a daughter having normal color vision”, it is a given that the child is a daughter. Only look at the 2 daughter possibilities = 2 out of 2 = 100%

Inheritance of Sex-Linked Traits A male can inherit colorblindness if his mother is a carrier of colorblindness. (Doesn’t matter father’s color vision, because males inherits Y from dad). A female must inherit the colorblindness allele from both parents. Mother must be at least a carrier. Father must be colorblind.

Polygenic Traits that are controlled by multiple genes. Examples: Human Height Skin Color Traits exhibit a greater range of variation