Human Heredity

Note Expectations:◦Cell phones and electronics are not

in use.◦You are taking the notes.◦You are helping the people at your

table to answer the questions.◦You are prepared to answer the

questions.

I can describe the chromosomes found in a human cell.

I can explain why a karyotype is useful.

I can explain how the sex of a baby is determined.

I can give examples of sex linked traits.

I can explain why sex linked traits are more common in males.

How many chromosomes does a human cell have?

Where did your chromosomes come from?

A human cell has 46 chromosomes.

A karyotype is a picture of chromosomes lined up in pairs from largest to smallest.

Predict how a karyotype would be useful.

What are the sex chromosomes?Predict what you think the

purpose of a sex chromosome is. What combination of sex

chromosomes codes for a male? A female?

Karyotypes are useful for two reasons:

1. To determine the sex of a baby.2. To determine if the baby has a chromosomal disorder. Two of the 46 chromosomes in a

human cell are sex chromosomes.

Sex chromosomes determine the sex of an individual.

Females have the genotype XX.Males have the genotype XY.The X chromosome is much larger than the Y chromosome and carries a lot more information.

Who determines the sex of a baby, the male or female? Why do you think so?

Predict how you would fill in the Punnett Square to determine the sex of a baby.

What chromosome do you think is bigger, the X or Y? Why?

The X chromosome is much larger than the Y chromosome and carries a lot more information.

Males actually determine the sex of a baby.

Draw the Punnett Square:

Genes located on the X and Y chromosomes are called sex-linked genes.

Most sex linked genes are found on the X chromosome because it is larger.

There are more than 100 sex-linked genetic disorders found on the X chromosome.

If males only have one X chromosome, will dominant and recessive alleles matter? Why or why not?

Give an example of a sex-linked trait.

Males have just one X chromosome, so all X linked alleles are expressed.

Color blindness is a recessive allele that is much more common in males than females.

Females will only be color blind if they have 2 recessive alleles.

However, a male with just one recessive allele will be color blind.

Other sex-linked traits include: hemophilia and muscular dystrophy

The remaining 44 chromosomes in a human cell are autosomes.

Most human traits are polygenic, meaning they are controlled by many genes.

Are genes the only thing that determine your phenotype? Why or why not?

What other factors could influence your traits?

Many traits are also influenced by environmental factors like nutrition and exercise and are not simply genetic.

There are also chromosomal disorders.

When do chromosomes separate?

Predict what would happen if chromosomes did not separate correctly.

Most chromosomal disorders occur as a result of nondisjunction during meiosis.

Nondisjunction means not coming apart.

If nondisjunction occurs, various numbers of chromosomes may occur in gametes, resulting in a chromosome disorder.

Down’s syndrome is a result of an extra 21st chromosome.

There are also sex chromosome disorders where there is an extra or missing chromosome.

Review QuestionsWhat is a karyotype and why is it

useful?What are the two sex chromosomes?

What combination of sex chromosomes results in a male and female?

Explain why sex linked traits are more common in males.

What is nondisjunction? What is the result of nondisjunction?

Draw the Punnett Square to determine the sex of a baby.

PEDIGREE NOTES

Note Expectations:◦Cell phones and electronics are not

in use.◦You are taking the notes.◦You are helping the people at your

table to answer the questions.◦You are prepared to answer the

questions.

I can explain why pedigrees are useful.

I can draw a pedigree.I can determine whether a trait is

dominant or recessive based on a pedigree.

I can determine whether a trait is sex linked or autosomal based on a pedigree.

What is a pedigree?When you draw a pedigree, how

are males represented? Females?How do you show that someone

has a trait?

A pedigree is a chart of the genetic history of a family over several generations.

Females in a pedigree are represented by a circle.

Males in a pedigree are represented by a square.

Predict how a pedigree could be useful.

What are the sex chromosomes?What are the autosomal chromosomes?

Pedigrees can show an autosomal or an X-linked disease.

X-linked disorders are carried by the X chromosome.

Autosomal disorders are carried by one of the autosomal chromosomes.

Will males or females be more likely to have an autosomal disorder? Why?

Will males or females be more likely to have a sex-linked disorder? Why?

X-linked diseases show up mostly in males.

This is due to the fact that males have only one X chromosome.

If the disorder is found in a 50:50 ratio of males to females, it is autosomal.

Does a parent have to have a recessive trait in order for the children to have it? Why or why not?

Does a parent have to have a dominant trait in order for the children to have it? Why or why not?

If the disorder is dominant, one of the parents must have it.

If the disorder is recessive, the parents may not show the disorder.

How can a child end up with a recessive trait if neither of the parents have the trait?

Review: What does homozygous mean? Give an example.

Review: What does heterozygous mean? Give an example.

Recessive disorders can be hidden by parents if they have the heterozygous phenotype.

How to read a pedigree:Female with a trait Male with a trait   Married Children   Twins Female carrying a trait    Male carrying a trait

Review QuestionsWhat is a pedigree?Why is a pedigree useful?Why do sex-linked traits show up

more often in males?What is the ratio for an

autosomal disorder of male to female in a pedigree?

How is it possible for a child to have a recessive trait if neither of the parents have it? Explain your answer.

SEX LINKED TRAITS

Note Expectations:◦Cell phones and electronics are not

in use.◦You are taking the notes.◦You are helping the people at your

table to answer the questions.◦You are prepared to answer the

questions.

I can draw Punnet Squares for sex linked traits.

I can predict the outcome of genetic crosses involving sex linked traits.

I can explain why sex linked traits occur more often in males.

How many X chromosomes does a female have? To have a recessive trait, how many recessive alleles does a female need on the X chromosomes?

How many X chromosomes does a male have? To have a recessive trait, how many recessive alleles does a male need on the X chromosomes?

Females have 2 X chromosomes, so they must have 2 recessive alleles to show a recessive trait.

Males only have 1 X chromosomes, so they will show all recessive traits no matter what.

When doing punnett squares involving sex linked genes, the trait is written as a superscript.

What is a superscript?Predict how you would write an allele on the X chromosome.

Colorblindness is a recessive trait (cc)◦Normal Female:◦Female who is colorblind:◦Normal Male:◦Male who is colorblind:

You fill in the Punnett Square just like before, but now you use the X and Y chromosomes.

Why do you need to include the X and Y chromosome when doing Punnett Squares with sex linked traits?

Example: A colorblind male mates with a female who carries the trait but has normal vision. Show the Punnett Square for this cross.

Since females have two X chromosomes, one X chromosome is usually shut off in cells.

The shut off X chromosome forms a dense region in the nucleus known as a Barr body.

Are calico cats normally male or female? Why?

X chromosome inactivation is the reason you can have calico cats, and they are only female.

A female who is colorblind marries a man with normal vision. Colorblindness is a recessive trait. What percent of their children will be colorblind?

Hemophilia is a recessive trait that does not allow a person’s blood to clot. A normal man marries a woman who is heterozygous for the trait. What percent of their children will have hemophilia?

ReviewDemonstrate how you would write the

genotype for a female who is colorblind. Remember that colorblindness is found on the X chromosome and is recessive. 

What chromosomes are sex linked disorders found on?

How many recessive alleles does a female need to have a sex linked disorder? Why? 

How many recessive alleles does a male need to have a sex linked disorder? Why?

What is X chromosome inactivation? Give an example.