Chapter 4, Section 1

Traits are controlled by: A single gene with 2 alleles. OR A single gene with multiple alleles. OR Many genes that act together. Patterns of Human Inheritance

Single genes with 2 alleles: You learned about this. This is when a trait is controlled by two alleles that have completely different phenotypes. For example, the gene for a widow’s peak is controlled by 2 alleles. You either have a widow’s peak or you don’t. So, you could have any combination of W and w. Remember this? Patterns of Human Inheritance

Single genes with multiple alleles: Some traits are controlled by one gene that has more than 2 alleles. For example, blood type is controlled by one gene with 3 alleles. Your alleles could be I A, I B, a combination of the two, or ii. So, there is one gene (the letter I) and three alleles (I A, I B, or i). Patterns of Human Inheritance

Traits that are controlled by M-A-N-Y genes Height and skin color are traits that are controlled by many genes. The genes work together as a group to produce a single trait. Patterns of Human Inheritance

You have 23 pairs of chromosomes in your body. One of those pairs is the sex chromosome that carries genes that determine whether you are a boy or a girl. The sex chromosomes carry genes that determine other traits too. The Sex Chromosomes

Girl- XX Boy- XY Sex chromosomes are the only chromosomes that don’t always match. The Sex Chromosomes

Sex Chromosomes and Fertilization All eggs carry X chromosomes because that is all women have (XX). Half of a man’s sperm cells carry X chromosomes and the other half carry Y chromosomes because men have X and Y. The Sex Chromosomes

Where do baby girls come from? When an X sperm cell fertilizes an egg a baby girl will be formed (2 X chromosomes). Where do baby boys come from? When a Y sperm cell fertilizes an egg a baby boy will be formed (X chromosome from egg, Y chromosome from sperm) The Sex Chromosomes

Sex-Linked Genes Genes on the X and Y chromosomes are often called sex- linked genes. Why? Because the alleles of the genes are passed from parent to child on a sex chromosome. The Sex Chromosomes

Sex-Linked Genes Sex linked traits are traits that are controlled by sex- linked genes. A sex-linked trait is red-green colorblindness. The Sex Chromosomes

Sex-Linked Genes Recall: male sex chromosomes don’t match (XY), so X and Y chromosomes each have different genes. That means that an allele on the X chromosome may not have a matching allele on the Y chromosome. The Sex Chromosomes

Sex-Linked Genes Sex-linked genes have dominant and recessive alleles, just like other genes. In females, the dominant gene on one X will mask a recessive allele on the other X. In males, since X and Y don’t match, even if there is a recessive allele the trait will show up. The Sex Chromosomes

The trait for colorblindness is controlled by a RECESSIVE allele on the X chromosome. A carrier for a trait is someone who has one recessive allele and one dominant allele for a trait, but doesn’t have the trait. A carrier for colorblindness is not colorblind, she is someone who can pass the trait on to offspring. Colorblindness

Many more males than females have colorblindness. WHY??? Because…the gene for color vision is on the X chromosome, not on the Y chromosome, so….. There isn’t an allele on the Y chromosome that will mask the colorblindness allele on the X chromosome. Colorblindness

Many of a person’s characteristics are determined by an interaction between genes and the environment. For example, a person’s height can be influenced by diet. Skills can also be affected by environmental factors, such as playing a musical instrument. The Effect of the Environment