Chapter 4: Modern Genetics

Section 1: Human Inheritance

Traits Controlled by Single Genes

• Many human traits are controlled by a single gene (one dominant allele, one recessive allele)– These traits have two distinct phenotypes– Ex: Widow’s peak, smile dimples

Multiple Alleles

• Some human traits are controlled by a single gene that has more than two alleles

• Multiple alleles – 3 or more forms of a gene that code for a single trait

• Even if a gene has multiple alleles, each person can only carry 2

• Ex: Blood type – A, B, AB, or O

Traits Controlled by Many Genes

• Some human traits show a large number of phenotypes because the traits are controlled by many genes which act together to produce the one trait

• At least 4 genes control height• At least 3 genes control skin color• This is why there are so many variations in

these two traits

The Effect of Environment

• Effects of genes are often changed by the environment

• Ex: height – diet poor in nutrients can prevent a person from growing to their full height

Male or Female?

• The sex chromosomes are the only pair of chromosomes that don’t always match

• XX = female, XY = male

• All eggs from the mother are X

• Sperm from the father can be either X or Y– If an X sperm matches with the X egg, the baby

will be a girl– If a Y sperm matches with the X egg, the baby

will be a boy

Sex-Linked Genes

• Some traits occur more often in one sex than the other because they are carried on the sex chromosomes

• Sex-linked genes – Genes on the X and Y chromosome whose alleles are passed from parent to child on a sex chromosome

• A dominant female will mask a recessive allele

Sex-Linked Genes

• Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessive allele

• Ex: Colorblindness• Carrier – a person who has one recessive

allele for a trait and one dominant allele– Does not exhibit the trait, but can pass it on– Only females can be carriers for sex-linked

traits

Pedigrees

• Pedigree – a chart or “family tree” that tracks which members of a family have a particular trait

• Different symbols represent males and females and different shadings represent the presence of a trait