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. This may be because: The genes are sex-linked. Incomplete dominance Codominance Multiple alleles Incomplete dominance in snapdragons produces pink flowers from red and white parents 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. CrCr CrCw CwCw

Flower Color in Snapdragons Red flower White flower Parents CrCr X CwCw Gametes Cr Cw Possible fertilizations Pink F1 offspring CrCw

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 F1? 2. If two of the F1 offspring were crossed, what genotypes and phenotypes ratios would appear in the F2?

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 White cow Red bull CRCR CWCW Parents X CR CW Gametes Possible fertilizations Roan CRCW F1 offspring

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

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

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: IA, IB, i (however any one individual can possess only two alleles) IA & IB are co-dominant and i is recessive.

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

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

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: 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. Female Male XX XY X Parents X X X Y Gametes Possible fertilizations XX XY XX XY Offspring Sex: Female Male Female Male

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

Examples: X-linked traits are denoted XD for a dominant allele and Xd for a recessive allele For most X-linked genes, the dominant form is the healthy form. Hemophilia Xh Normal blood clotting XH Muscular dystrophy Xm Non-muscular dystrophy XM Color blindness Xĉ 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 Sex-linked traits show a distinct pattern of inheritance. Fathers pass sex-linked alleles to all their daughters but not to their sons. 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. In contrast, any male receiving the recessive allele from his mother will express the trait. Carrier mother X X Y Unaffected father Carrier daughter X Affected son X Y Unaffected son Y X Unaffected daughter X

Practice Problems 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? One of the daughters from the above problem marries a color blind man. What proportion of their sons will be color blind?