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