genetic probabilities

GENETIC PROBABILITIES

SEX CHROMOSOMES

TYPES OF CHROMOSOMES

• Just like our cells are categorized as either normal body cells (somatic cells), our chromosomes are categorized the same way.



• Chromosomes (DNA) that code for all your non-sex traits are called “autosomesautosomes” and those are chromosomes 1 - 22chromosomes 1 - 22.



• Chromosomes (DNA) that code for all your non-sex traits are called “autosomesautosomes” and those are chromosomes 1 - 22chromosomes 1 - 22.

• Chromosome 23 Chromosome 23 is our “sex chromosomesex chromosome” and it codes for all of our sex/gender (male/female) traits.

SEX CHROMOSOMES

• Chromosome 23 from each parent are the only chromosomes that are not homologous pairs.

SEX CHROMOSOMES

• Chromosome 23 from each parent are the only chromosomes that are not homologous pairs.

• They come in 2 forms (shapes), one that looks like the normal “X” shape and one that is missing much of the bottom part of the legs so it resembles a “Y”.

X chromosome

Y chromosome

CHROMOSOME GENOTYPES

• Males have one of each type of sex chromosome, an X and a Y so their genotype is referred to as “XY” even though we’re talking about whole chromosomes, not just alleles on a chromosome.

CHROMOSOME GENOTYPES

• Males have one of each type of sex chromosome, an X and a Y so their genotype is referred to as “XY” even though we’re talking about whole chromosomes, not just alleles on a chromosome.

• Females have 2 X chromosomes so their genotype is referred to as “XX”.

A BABY’S GENDER• Remember that a mom only has X chromosomes to

donate to her baby. Dad can donate either an X or a Y to his baby, so dads determine the gender of the baby.



• Dad can donate an X that joins Mom’s X to make a baby girl ( ) XX



• Dad can donate an X that joins Mom’s X to make a baby girl ( ) XX

• Dad can also donate a Y that joins Mom’s X to make a baby boy ( ) XY

HERE’S WHY IT’S UP TO DAD

P = XY x XX dad mom

X YX XX XY XX = daughter = 50%X XX XY XY = son = 50%

SEX-LINKED TRAITS

• Look again at the photo of the X and Y chromosomes.

The X chromosome is much bigger and has room for extra genes (codes for traits).

The Y chromosome is much smaller and only has room for gender related genes.

Extra space for some non-gender traits like baldness, color vision and blood clotting.

SEX-LINKED TRAITS


• It’s like the X chromosome has an extra leg that the Y doesn’t have.




SEX-LINKED TRAITS


• It’s like the X chromosome has an extra leg that the Y doesn’t have.

• Because these extra genes are on a sex chromosome, they are called “sex-linked traits” (linked to the X)




SYMBOLS

• The symbols for the alleles that are sex-linked (linked to the X chromosome) follow all the same rules you’ve already learned.

SYMBOLS


• Normal color vision is dominant, colorblindness is recessive so the 2 alleles are represented by N and n.

SYMBOLS



• Hair is dominant, baldness is recessive so the alleles are represented by H and h.

SYMBOLS



• Hair is dominant, baldness is recessive so the alleles are represented by H and h.

• Blood clotting is dominant, hemophilia (your blood doesn’t clot) is recessive so the symbols are B and b.

SYMBOLS

• The difference is that you show they are linked to the X chromosome, you put the symbols as a superscript on the superscript on the X: X:

SYMBOLS


XN = normal color vision (dominant) on the XXn = colorblindness (recessive) on the X

SYMBOLS



XH = hair (dominant) on the XXh = baldness (recessive) on the X

SYMBOLS



XH = hair (dominant) on the XXh = baldness (recessive) on the X

XB = blood clotting (dominant) on the XXb = hemophilia (recessive) on the X

GENOTYPES AND PHENOTYPES

• Since women have a genotype of XX, sex-linked traits for them behave normally:



• the only way for a woman to express a recessive trait on her X chromosome is if she is purebred / homozygous for it.




XNXN or XNXn = normal visionXnXn = colorblindness




XNXN or XNXn = normal visionXnXn = colorblindness

* A girl gets an allele from each parent on the X chromosome.


• Since men have a genotype of XY, sex-linked traits for them behave differently:



• the trait is only on the X not on the Y. Therefore, whatever he receives from his mom is going to be expressed. There is no matching allele on the Y chromosome from his dad.



• the trait is only on the X not on the Y. Therefore, whatever he receives from his mom is going to be expressed. There is no matching allele on the Y chromosome from his dad.

XNY = normal visionXnY = colorblindness



• the trait is only on the X not on the Y. Therefore, whatever he receives from his mom is going to be expresses. There is no matching gene on the Y chromosome from his dad.

XNY = normal visionXnY = colorblindness

• A boy gets only 1 allele in a sex-linked trait and it’s from his mom on the X chromosome.

Test your knowledge (and your family tree)

• Most types of baldness are sex-linked traits.



• If “hair” is dominant and baldness is recessive, what would a male’s genotype be if he had hair?



• If “hair” is dominant and baldness is recessive, what would a male’s genotype be if he had hair? XHY




• What would a bald male’s genotype be?




• What would a bald male’s genotype be? XhY





Question: If a boy’s father is bald, does the boy have to worry about going bald when he is older?





Question: If a boy’s father is bald, does the boy have to worry about going bald when he is older?

NO! He is a boy because he received a Y from his dad that does not carry that trait!

A boy can only get a sex-linked trait from his mother.

A boy can only get a sex-linked trait from his mother.

Therefore, if he is wondering if he’ll go bald when he’s older, he needs to look at the males on his mother’s side of the family.

A bald uncle or grandfather on Mom’s side of the family means there is a good chance the boy will go bald when he gets older.

A bald uncle or grandfather on Dad’s side of the family is nothing to worry about for the boy!

Now it’s time to write your summaries – there’s one on both sides of your paper.

When you’re done, do your practice problems –

Use your notes to help you with the symbols and Use your notes to help you with the symbols and genotypes/phenotypes.genotypes/phenotypes.

genetic probabilities

Documents