Human InheritanceHuman Inheritance

Key Concepts1.What are some patterns of inheritance in humans?2.What are the functions of the sex chromosomes?3.What is the relationship between genes and the environment?

Key TermsKey TermsMultiple allelesSex chromosomesSex-linked genescarrier

Patterns of Human InheritancePatterns of Human InheritanceHuman 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 AllelesSingle Gene with Two AllelesHave 1 dominant and 1 recessive alleleHave 2 distinctly different phenotypesEx. Widow’s Peak◦Smile dimples – Draw a Punnett Square that

shows a cross between two heterozygotes for smile dimples

Single Genes with Multiple AllelesSingle 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 GenesTraits Controlled by Many GenesProduce a large variety of phenotypesGenes act together as a group to

produce a single traitEx. Height – at least 4 genes◦Skin color – at least 3 genesQuestion: 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 ChromosomesThe Sex ChromosomesOne 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?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 FertilizationSex Chromosomes and Fertilization

Egg cells all carry an X chromosomeSperm cells carry either an X or a YWhen 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 GenesSex-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 colorblindnessMost of the genes on the X chromosome

are not on the Y chromosomeAn allele on the X may not have a match

on the Y

Sex-Linked GenesSex-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 ColorblindnessInheritance of ColorblindnessColorblindness is a trait controlled by a

recessive allele on the X chromosomeMany more males have colorblindness

than femalesFemales 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 ColorblindnessRed-Green Colorblindness

Inheritance of ColorblindnessInheritance of ColorblindnessIf 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 ColorblindnessTest for Red Green Colorblindness

Inheritance of ColorblindnessInheritance of Colorblindness

XCY

XC

Xc

MotherXCXc

Father XCY

XCXC XCY

XCXc XcY

The Effect of EnvironmentThe Effect of EnvironmentMany 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