Human Inheritance

Human Inheritance Key Concepts What are some patterns of inheritance in humans? What are the functions of the sex chromosomes? What is the relationship between genes and the environment?

Key Terms • Multiple alleles • Sex chromosomes • Sex-linked genes • carrier

Patterns of Human Inheritance • Human traits are controlled by: • single genes with two alleles • others by single genes with multiple alleles. • Still other traits are controlled by many genes that act together

Single Gene with Two Alleles • Have 1 dominant and 1 recessive allele • Have 2 distinctly different phenotypes • Ex. Widow’s Peak • Smile dimples – Draw a Punnett Square that shows a cross between two heterozygotes for smile dimples

Single Genes with Multiple Alleles • Some traits have more than 2 alleles although a person can have only 2 of those alleles because chromosomes exist in pairs. Each chromosome in a pair carries only 1 allele for each gene • Ex. Human blood type – 3 alleles A, B, O A and B are codominant O is recessive

Traits Controlled by Many Genes • Produce a large variety of phenotypes • Genes act together as a group to produce a single trait • Ex. Height – at least 4 genes • Skin color – at least 3 genes Question: In our face trait lab most of the children had dark colored skin. What was the assumption about the parents that caused this result?

The Sex Chromosomes • One of the 23 pairs of chromosomes that carries the genes that determine whether a person is male or female • Also carries genes that determine other traits

Girl or Boy? • The only chromosome pair that does not always match • Girl – two sex chromosomes match (X,X) • Boy –don’t match, one is an X other is a Y • The Y chromosome is smaller than the X

Sex Chromosomes and Fertilization • Egg cells all carry an X chromosome • Sperm cells carry either an X or a Y • When a sperm cell with an X chromosome fertilizes an egg cell, the egg has 2 X’s = girl • When a sperm cell with a Y chromosome fertilizes an egg cell, the egg has an X and a Y = boy

Sex-linked Genes • Genes for some human traits are carried on the sex chromosomes • Traits controlled by sex-linked genes are called sex-linked traits • EX. Red-green colorblindness • Most of the genes on the X chromosome are not on the Y chromosome • An allele on the X may not have a match on the Y

Sex-Linked Genes • Sex-linked genes can have dominant and recessive alleles • In females a dominant allele on one x will mask a recessive on the other X • In males, there is no matching allele on the Y to mask a recessive allele on the X • Any trait on the X chromosome in males (even a recessive trait) will produce the trait in a male who inherits it.

Inheritance of Colorblindness • Colorblindness is a trait controlled by a recessive allele on the X chromosome • Many more males have colorblindness than females • Females can be carriers. They have one dominant trait for normal color vision and one recessive trait for colorblindness. They have normal color vision

Red-Green Colorblindness

Inheritance of Colorblindness • If the mother passes on the X chromosome to a son, he will be colorblind • If she passes it onto a daughter, she will also have an X chromosome from her father. If her father has normal color vision, then she will inherit the dominant allele for normal color vision from him and she will have normal color vision

Test for Red Green Colorblindness

Inheritance of Colorblindness Father XCY XC Y XC XCXC XCY Mother XCXc XCXc XcY Xc

The Effect of Environment • Many of a person’s characteristics are determined by an interaction between genes and the environment • Several genes work together to determine height • Environment affects height – a poor diet can prevent a person from reaching their potential • Environmental factors can also affect human skills – Ex. Playing a musical instrument – need instruction to play well

Human Inheritance