Heredity Chapter 12 part 2 Beyond Mendels Laws

Heredity Chapter 12, part 2

Beyond Mendel’s Laws • Not all traits are controlled by single genes with dominant and recessive alleles. Other patterns of heredity involve: • Incomplete dominance • Sex-linked traits • Multiple alleles • Multiple genes (most traits involve this) • Chromosomal abnormalities

Incomplete Dominance • In incomplete dominance and codominance, both alleles are expressed in the phenotype. • incomplete dominance: two traits appear to blend in the heterozygotes. • co-dominance: both traits appear in the heterozygotes.

These snapdragons have two alleles controlling flower color: R (red) and R’ (white). Heterozygotes (RR’) have pink flowers. Unlike the case in complete dominance, the phenotypes show us which plants are heterozygous.

mother C 1 C 2 C 1 father C 1 C 2 eggs C 2 C 1 sperm In humans, a gene affecting hair texture (curly, wavy, straight) shows incomplete dominance. C 1 C 1 C 2 C 2 C 2

The golden palomino horse is a cross between a white and a brown horse. This is another example of incomplete dominance: the colors appear to blend in the horse’s hairs.

The red roan horse has both white and red -brown hairs, while the blue roan has both white and gray hairs. The coat colors of both parents are expressed in the hairs. This is co-dominance. At the gene level, incomplete dominance and codominance are the same: in both cases, both alleles are expressed in the heterozygote. The only difference is at the phenotype level.

Solving single-gene (monohybrid) crosses with incomplete dominance. One hair color in cattle is controlled by a gene that produces red (R) or white (R’) hairs. Heterozygotes (RR’) are roan. a. What color would the offspring of a red bull and a white cow be? b. What are the phenotypic ratios of a cross between a white cow and a roan bull?

Sex-Linked Traits • In humans, women have two X chromosomes while men have an X and a Y. • Traits that are carried on the sex chromosomes will show different genotypic and phenotypic rations in men and women. • The X chromosome has many genes, while the Y has only a few, so there are many more X-linked traits than Y-linked traits.

Women pass their X chromosomes to their children. Men can contribute either an X or a Y. Which parent determines the sex of the child? Mother or father? Can men be carriers of a recessive X-linked trait? If a boy has an X-linked trait, which parent did he inherit the trait from?

female parent eggs X 1 Xm male parent Xm Y sperm When determining the outcome of a cross that involves an Xlinked trait, we have to take into account how the two sex chromosomes are distributed in the offspring. Xm Y X 2 Xm female offspring X 1 Y X 2 Y

This diagram illustrates a cross for an X-linked trait in fruit flies. Red eyes are dominant, white are recessive. R female parent R XR Xr r XR Xr R r eggs XR R Xr R sperm male parent XR Y XR XR female offspring R Xr XR r all the F 2 females have red eyes half the F 2 males have red eyes, half have white eyes

Red-green color blindness is an X-linked trait. Charts such as these are used to diagnose red-green color blindness. They look very different for those with normal vision and those with RG-color blindness. Normal RG color-blind A 29 70 B 45 nothing C abstract 5 D 26 nothing

Solving X-linked crosses. Red-green color-blindness is X-linked. If a man and woman with normal vision have a color-blind son: a. What are the genotypes of the parents? b. What are the odds of having another color-blind son? c. What are the odds of having a color-blind daughter?

Multiple Alleles • Human blood type (A, B, AB, and O) is determined by a gene that has three alleles. • A and B are co-dominant • O is recessive to both • Though there are three alleles, each person still has only two copies of the ABO gene.

A and B alleles produce A and B proteins on the surface of red blood cells. The O allele produces neither of these proteins.

Multiple Genes • Most human traits are the result of multiple genes. • In some cases (such as skin and hair color), there are multiple copies of the same gene (such as the melanin gene). • In many others, there are many different genes controlling a trait, and the environment may affect how a trait is expressed. (Example: human height)

Human skin color is controlled by at least three melaninproducing genes, which are incompletely dominant. sperm This massive Punnet square shows a cross between two people who are heterozygous for all three genes. eggs

Chromosomal Abnormalities • Chromosomal abnormalities include: • nondisjunctions (failure of chromatids to separate during meiosis) • deletions of parts of chromosomes • Most chromosomal abnormalities cause cell death, but a few are survivable.

Cri-du-chat syndrome is caused by a deletion of a large segment of chromosome 5. Cri-du-chat children often have small head circumference and are severely cognitively challenged. Some may have heart defects, muscular or skeletal problems, or vision problems.

Trisomy 21 results in Down Syndrome. Using what you know about meiosis, explain how a fertilized human egg cell can end up with three copies of chromosome 21. If a person with Down Syndrome planned to have a child, could the child inherit Down Syndrome?

Nondisjunction of the sex chromosomes is more often survivable than nondisjunctions of somatic chromosomes. As long as the fetus has at least one X chromosome, it can survive.

Klinefelter syndrome produces an XXY male. At puberty, Klinefelter males fail to fully develop secondary sex characteristics. Men with this syndrome may or may not be sterile. They have a tendency to gain weight easily and their muscle mass is underdeveloped, but mental function is usually normal.

Turner syndrome occurs when a girl inherits only one X chromosome. Turner children are often short, and may show swelling in the hands and feet. Some have heart defects, but most are cognitively normal. Hormone therapy at puberty can help Turner girls grow to normal height and develop secondary sex characteristics.

Recap • Traits inherited by classic Mendelian genetics are by far in the minority. • Incomplete dominance, sex linkage, and multiple alleles involve single-gene traits that show unique patterns of inheritance. • Most traits involve multiple genes and gene-environment interactions.