Incomplete Dominance, Codominance, Sex-linked and Polygenic Inheritance

Mendel’s Dominance Mendel’s rule of dominance was complete dominance Homozygous dominant organisms have the dominant phenotype Heterozygous organisms have the dominant phenotype Example: Both PP and Pp plants have the dominant PURPLE phenotype (P=purple and p=white flowers)

Incomplete Dominance R F1 generation Rr r All Rr = pink Offspring have an appearance somewhat in between the phenotypes of the two parents and neither allele is dominant. “Mixed” Blended r R All Rr = pink (heterozygous pink) F1 generation Rr

Incomplete Dominance Incomplete dominance occurs when two or more alleles influence the phenotype Neither allele is completely dominant over the other

Incomplete Dominance

Codominance BOTH alleles are expressed equally in heterozygous individuals. Use two capital letters for both alleles. Neither allele is dominant over the other. Usually phenotypes are stripes or spots.

YOU tell me which type of dominance… Codominance!

Type of Dominance? Incomplete Dominance!

Type of Dominance? Incomplete Dominance!

Codominance Example: blood type Determined by whether or not you have A or B proteins IA = A protein IB = B protein I = no protein

Codominance

Codominance Problem Example:homozygous male Type B (IBIB) x heterozygous female Type A (IAi) IA IB i IAIB IBi 1/2 = IAIB 1/2 = IBi

Another Codominance Problem Example: male Type O (ii) x female type AB (IAIB) i IA IB IAi IBi 1/2 = IAi 1/2 = IBi

Codominance Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? boy - type O (ii) X girl - type AB (IAIB)

Codominance Answer: IB IA i IAIB ii Parents: genotypes = IAi and IBi phenotypes = A and B

Sex-linked Traits Traits (genes) located on the sex chromosomes Sex chromosomes are X and Y XX genotype for females XY genotype for males Many sex-linked traits carried on X chromosome

Sex – Linked Traits Example: Colorblindness If the mother carries the colorblindness gene on her X chromosome, her son could get it. As long as one X chromosome is ok, a female will not express the trait XC X y XcX X y X X X y

Sex-linked Traits Example: Colorblindness Sex Chromosomes XX chromosome - female Xy chromosome - male Color- blindness

Polygenic Inheritance Inheritance pattern controlled by two or more genes Genes can be on the same chromosome or on different chromosomes Genotypes written as AA, Aa, or aa except you will have three or more genes each with a dominant and recessive trait Skin color is influenced by the additive effects of melanin by three to six genes. Eye color is also a result of melanin added from multiple genes.

Polygenic Inheritance of Skin Color Gametes ABC ABc AbC Abc aBC aBc abC abc AABBCC AABBCc AABbCc AaBBCC AAbbcc aaBBCc aaBbcc The table shows typical ranges of skin color combinations. Other polygenic influenced conditions include Spina bifida and Cleft palate.