1. 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?

2. Key Terms
Multiple alleles
Sex chromosomes
Sex-linked genes
carrier

3. 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

4. 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

5. 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

6. 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?

7. 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

8. 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

9. 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

10. 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

11. 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.

12. 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

13. Red-Green Colorblindness

14. 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

15. Test for Red Green Colorblindness

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

17. 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