Bio 12Bio 12

Review: Sex Chromosomes

Humans have 23 pairs of chromosomes. 22 of them are called autosomal. The 23rd pair are the sex chromosomes. The sex chromosomes determine whether an

offspring will be male or female.o XX = Femaleo XY = Male

Review: Females Have two x chromosomes (one from

mother and one from father)

Have one x chromosomes (from mother) and one y (from father)

Review: Males

Genes that are carried by either sex

chromosome are said to be sex linked. Sex linked genes have inheritance patterns

that differ from autosomal geneso Since males and females differ in their sex

chromosomes, inheritance patterns are different for males and females.

What is sex-linked inheritance?

The study of inheritance of

genes located on sex chromosomes was pioneered by T. H. Morgan and his students at the beginning of the 20th century.

He used Drosophila melanogaster (fruit flies) to study genetics

Although Morgan studied fruit flies, the same genetic principles apply to humans.

History…..

Morgan noticed that

chromosomes of male and female fruit flies were slightly different

He is credited as the first person to see the physical difference in the x and y chromosomes!

More history…..

Knowing red eyes are

dominant and white eyes are recessive, Morgan crossed whited eyed with red eyedo F1’s were what he

expected 3 red: 1 white

But in F2, he found all the white eyed flies where male!o He predicted that the eye

colour was connected with the sex of the fruit fly!

More history…..

Use X or Y to represent sex chromosome and

capital or lowercase letters to represent dominant or recessive allele

Examples:• XG XG

• XG Xg

• Xg Xg

• XG Y• Xg Y

Genotype notation:

- X linked diseases are those for which the

gene is present on the X chromosome. Because of this, males and females sho

w different patterns of inheritance and s everity of disease .

T here are both dominant and recessive- X linked diseases

-X linked

- X linked genes are never passed from father to

son. Males are never carriers – if they have a mutate

d gene on the X chromosome, it will be expressed …. Why?o The Y chromosome is the only sex chromosome t

hat passes from father to son……does not pass on an X!

-X linked Characteristics:

- X linked dominant diseases are those that are e

xpressed in females when only a single copy of the mutated gene is present.

- Very few X linked dominant diseases have bee n identified (e.g. hypophosphatemic rickets, Al

port syndrome, diabetes insipidus)

- X linked dominant

Affected females produce 50% normal and 50%

affected offspring…….. heterozygous Females are more likely to be affected. Sinc

e females have 2 X chromosomes, they have 2 “chances” to inherit the mutated allele.

- X linked dominant diseases:

The pattern for the pedigree of X-linked dominant inheritance:

Note: If father is affected, all daughters will be affected

- X linked recessive diseases are those in whi

ch a female must have two copies of the mu tant allele in order for the mutant phenotyp

e to develop.o Trait skips generationso Affected fathers DO NOT pass to their sonso Males are more often affected than females

Includes: color blindness, hemophilia, and Duchenne muscular dystrophy.

- X linked recessive

The pattern for the pedigree of X-linked recessive

inheritance:

In humans there are very few traits that

are sex linked on the “Y” chromosomes These traits are only expressed in males Transmitted from father to son Example: gene SRY (triggers testis

development )

Note: We will not be doing any y-linked genetics problems in Bio 12!

Y….

X-linked Genes WS Page 167 #6, 7

To do: