1. Chapter 4: Modern Genetics
Section 1: Human Inheritance

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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