Patterns of Human Inheritance
Some human traits are controlled by single genes with two alleles, while others by single genes with multiple alleles. Still other traits are controlled by many genes that act together.
Single Genes With Two Alleles
Many human traits are controlled by a single gene with one dominant allele, and one recessive. These human traits have two distinctively different phenotypes.
Single Genes With Multiple Alleles
Some human traits are controlled by a single gene that has more than two alleles. Such a gene is said to have multiple alleles- three or more forms of a gene that code for a single trait.
Blood Type
Human blood type is controlled by a gene with multiple alleles. There are four main blood types: A,B,AB, and O. Three alleles control the inheritance of blood types. The allele for blood type A and the allele for blood type B are codominant. The allele fo
Traits Controlled by Many Genes
Some human traits show a large number of phenotypes because the trait is controlled by many genes. The genes act together to as a group to produce a single trait.
The Sex Chromosomes
The sex chromosomes are one of the 23 pairs of chromosomes in each body cell. The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits.
Girl or Boy?
The sex chromosomes are the only pair that do not always match. Girls' sex chromosomes match, but boys' don't. Girls have an X and an X, and boys have an X and a Y.
Sex Chromosomes and Fertilization
What happens to the sex chromosomes when the egg and sperm cells form? Since both a female's sex chromosomes are Xs, all eggs carry one X chromosome. Males are a mix, so half the sperm cells carry X, and half Y. When a sperm cell fertilizes an egg, the eg
Sex-Linked Genes
Genes on the X and Y chromosomes are often called sex-linked genes, because their alleles are passed down from parent to child on a sex chromosome. Traits controlled by a sex-linked chromosome are called sex-linked traits. One sex-linked trait is red-gree
X and Ys
The X and Y chromosomes have different genes. Most of the genes on the X are not on the Y. Therefore, an allele on an X may have no corresponding allele on the Y.
Inheritance of Colorblindness
Colorblindness is a trait controlled by a recessive allele on the X chromosome. Many more males then females have colorblindness. A carrier is a person who has one recessive allele for a trait and one dominant. The carrier can pass the trait down to its o
The Effect of Environment
In some organisms, the effects of genes can be influenced by the environment. Many of a persons characteristics are determined by an interaction between genes and the environment. People's diets can affect their heights.