Allele Expression Allele expression not always as simple

Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together and give rise to phenotypes that are dissimilar to both of the parents. Incomplete dominance in snapdragons produces pink flowers from red and white parents This may be because: The genes are sex-linked. Incomplete dominance Codominance Multiple alleles Roan coat color in cattle is a result of codominance between red and white alleles

Incomplete Dominance In cases of incomplete dominance, neither allele dominates and the heterozygote is intermediate in phenotype between the two homozygotes. Examples of incomplete dominance include flower color in snapdragons (right) and sweet peas, where red and white flowered plants cross to produce pink flowered plants. Cw Cw C r

Flower Color in Snapdragons Red flower White flower Parents X Cr Cr Cw Cw Gametes Cr Cr Cw Cw Possible fertilizations F 1 offspring Cr Cw Pink Cr Cw Pink

Example problems (answer by showing Punnet squares): 1. If a white flowered plant is crossed with a red flowered plant, what are the genotypic and phenotypic ratios of the F 1? 2. If two of the F 1 offspring were crossed, what genotypes and phenotypes ratios would appear in the F 2?

Codominance In cases of codominance, both alleles are independently and equally expressed in the heterozygote. Roan (stippled red and white) coat color in cattle. AB human blood groups. Black and tan tabby cats Red bull White cow CW CW CR CR Parents X Gametes CR CR CW CW Possible fertilizations CR CW Roan F 1 offspring

Example problem 1. Cross two heterozygous (roan) shorthorn cattle Roan bull Roan cow CR CW X Parents Gametes CR CR CW CW CR CW Possible fertilizations CR CR CR CW Red Roan CR CW CW CW Offspring Roan White

Example Problem 2. A true breeding red parent is crossed with a roan parent Roan cow Red bull Parents Gametes CR CW CR CR X CR CR CR CW Possible fertilizations CR CR CR CW CR CR Red Roan Red CR CW Offspring Roan

Multiple Alleles in Blood Humans have 4 blood group phenotypes: A, B, AB and O The four common blood groups of the human ABO blood group system are determined by three alleles: A B I , i (however any one individual can possess only two alleles) IA & IB are co-dominant and i is recessive.

Multiple Alleles in Blood group: AB EXAMPLE 1: Cross two parents, both with AB blood type Parent genotypes Gametes Blood group: AB X AB IA IB Possible fertilizations Children's genotypes Blood groups I AI A I AI B I BI B A AB AB B

Multiple Alleles in Blood group: B EXAMPLE 2: Two parents with blood groups A and B respectively, both heterozygous Blood group: A Parent genotypes Gametes X I Bi IB i I Ai IA i Possible fertilizations Children's' genotypes Blood groups I AI B AB I Bi B I Ai A ii O

XY Sex Determination In the XY type, sex determination is based on the presence or absence of the Y chromosome; without it, an individual will develop into a female. XY sex determination occurs in: Female Mammals (including humans) Fruit fly Drosophila Some dioecious (separate male and female) plants such as kiwifruit. Females are homogametic with two similar sex chromosomes (XX). The male has two unlike chromosomes (XY) and is heterogametic. Primary sex characteristics are initiated by genes on the X. ‘Maleness’ is determined by the Y. Gametes X XX Parents X X XY X Y Possible fertilizations Offspring Sex: XX Female XY XX Male Female XY Male

Sex Linkage Genes located on the X chromosomes are called X-linked X Y Because the Y chromosome is small and does not contain many genes, few traits are Y-linked and Y-linked diseases are rare. Note the size differences between the X and Y chromosomes. The Y lacks alleles for many of the genes present on the X.

Examples: D for a dominant X-linked traits are denoted X allele and Xd for a recessive allele For most X-linked genes, the dominant form is the healthy form. Hemophilia Xh Muscular dystrophy Xm Color blindness Xĉ Normal blood clotting XH Non-muscular dystrophy XM Non-color blindness XC

• A human female can be homozygous or heterozygous with respect to sex-linked genes. • Female heterozygous for X-linked alleles are called carriers, because they don’t have the disease (they have one good copy of the gene) but they do “carry” the bad allele. • Since (normally) males only have one X chromosome, males only have one copy of genes located on the X chromosome.

Sex Linkage Unaffected father Sex-linked traits show a distinct pattern of inheritance. Carrier mother Fathers pass sex-linked alleles to all their daughters but not to their sons. X Mothers can pass sex-linked alleles to both sons and daughters. In females, sex-linked recessive traits will be expressed only in the homozygous condition. Y XY X XX XY In contrast, any male receiving the recessive allele from his mother will express the trait. Unaffected son Unaffected daughter Carrier daughter Affected son

Practice Problems 1. Suppose a color blind man fathers children with a woman of the genotype XC XC. What proportion of daughters would be color blind? What proportion of sons would be color blind? 2. One of the daughters from the above problem marries a color blind man. What proportion of their sons will be color blind?