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 , 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
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 (XX) • Boy –don’t match, one is an X other is a Y (XY) • The Y chromosome is smaller than the X
Sex Chromosomes and Fertilization • Egg cells all carry an X chromosome • Sperm cells carry either an Xor 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, X……. result---------XY = 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 • Males cannot be carriers. If a male inherits one recessive allele – he will be colorblind.
Inheritance of Colorblindness • If the mother passes on the X chromosome- with the allele for colorblindness to a son, he will be colorblind- Males only have to inherit oneallele to be colorblind • If the mother passes the X chromosome with the colorblind allele 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- but she will be a carrier. Females need 2 to have trait.
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