13-1 Human Inheritance
Ms. De Los Rios
7 th Grade

1. Sex chromosomes – A pair of chromosomes carrying genes that determine whether a person is male or female.
2. Sex-linked geneA gene that is carried on a sex (X or Y) chromosome.
3. CarrierA person who has one recessive allele and one dominant allele for a trait.

Posted by: Julian
Location: Rockledge, Florida
I have noticed that there are a lot of physical traits that run in my family. It seems like poor eyesight is common. Both sets of my grandparents have poor eyesight and wear glasses. Both of my parents have poor vision and here I am, yes, I too have to wear glasses. Genetics rules.
What characteristics do you have that resemble those of your relatives? ____________________________________________
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Look at the students in your classroom. Some have curly hair and others have straight hair. Some people are tall, some are short and others are in between.
The different traits you see are determined by a variety of inheritance patterns.
Some human traits are controlled by single genes with two alleles, and others by single genes that have multiple alleles. Still other traits are controlled by many genes that act together.

Human traits that are controlled by a single gene with either a dominant or a recessive allele, such as a dimpled chin, or widow’s peak produce two distinctly different physical appearances, or phenotypes.

Some traits are controlled by a single gene that has more than two alleles (multiple alleles- 3 or more forms of a gene that code for a single trait).
Human blood type is controlled by a gene with multiple alleles .
There are 4 main blood types- A, B, AB, and O .
Three alleles control the inheritance of blood types.
type is codominant which means that both alleles for the gene are expressed equally. If you get an A allele from one parent and a B allele from the other you can have type AB blood.

Inheritance of Blood Type
The table shows which combinations of alleles result in each human blood type.
1. Interpret Tables- Genotypes are listed in the (left/right) column of the table, while phenotypes are on the (left/right).
2. Infer- Why are there more genotypes than phenotypes for blood types?
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Some human traits show a large number of phenotypes because the traits are controlled by many genes. The alleles of the different genes act together as a group to produce a single trait.
At least 4 genes control height in humans.
You can see the extreme range of heights in the picture.
Skin color is another human trait that is controlled by many genes.

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1a. Explain- Why do some traits exhibit a large number of phenotypes? _________________________________________
_____________________________________________________ b. Draw Conclusions- Aaron has blood type O. Can either of his parents have blood type AB? Explain your answer.
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I get it! Now I know that some human traits are controlled by_______
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The body cells of humans contain 23 chromosome pairs, or 46 chromosomes. The sex chromosomes are one of these pairs.
The sex chromosomes carry genes that determine a person’s gender as being either male or female. They also carry genes that determine other traits.
Girl or Boy?
The sex chromosomes are the only chromosome pair that doesn’t always match.
Girls have 2 X chromosomes XX
Boys have an X and a Y XY
The Y chromosome is much smaller than the X chromosome.

While all eggs carry one X chromosome, sperm cells carry either an
X or a Y chromosome. When an egg is fertilized by a sperm cell, the sex chromosome carried by the sperm cell determines whether a child will be a girl (X) or a boy (Y).
So…. The father determines the sex of the baby!!!!!!
Figure 3- Complete the Punnett square to show the possible genotypes and phenotypes of a child.
Determine- What is the probability that the child will be a girl? A boy?
Interpret Diagrams- What sex will the child be if a sperm with a Y chromosome fertilizes an egg.

Genes found on the X and Y chromosomes are often called sex-linked genes because their alleles are passed from parent to child on a sex chromosome. Traits controlled by sex-linked genes, such as colorblindness, are called sex-linked traits.
X and Y Chromosomes
The human X chromosome is larger and carries more genes than the human Y chromosome.
Sex-linked genes, like other genes, can have dominant and recessive alleles. In females, a dominant allele on an X chromosome will mask a recessive allele on the other X chromosome. But in males, because the Y chromosome is much smaller than the X chromosome, there is usually no matching allele on the Y chromosome to mask the allele on the X chromosome. So any allele on the X chromosome, even a recessive allele, will produce the trait in the male who inherits it.

Colorblindness is a trait controlled by a recessive allele on the X chromosome. Many more males than females have red-green colorblindness.
A carrier is a person who has one recessive allele for a trait and one dominant allele. A carrier of a trait controlled by a recessive allele does not express the trait, but can pass the recessive allele on to his or her offspring.
 In the case of sex-linked traits, only females can be carriers because they are the only ones who can carry two alleles for the trait.

Figure 6- Colorblindness Punnett Square pg. 449
Red-green colorblindness is a sex-linked trait.
1. Identify- Complete the Punnett square by filling in the child’s genotype, sex, and phenotype. For each child, draw the correct shape, and color it in to match the key.
2. Calculate- What is the probability that this Couple will have a colorblind child?
3. Apply Concepts- What allele combination would a daughter need to inherit to be colorblind?

2a. Review- What is the sex of a person who is a carrier for colorblindness? ___________________________________ b. ChallengeMary and her mother are both colorblind. Is Mary’s father colorblind, too? How do you know?________________
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I get it! Now I know that the functions of the sex chromosomes are ______________________________________________
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