bio 12. review: sex chromosomes humans have 23 pairs of chromosomes. 22 of them are called...
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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: