sex-linked inheritance x marks the spot!. sex chromosomes humans normally possess 23 pairs of...
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Sex Chromosomes cont’d Since females are XX, all female gametes carry an X chromosome. Male gametes may carry either an X or a Y (depending on chance). – So it is the sperm that determines the gender of the offspring.TRANSCRIPT
SEX-LINKED INHERITANCE
X Marks the Spot!
Sex Chromosomes• Humans normally possess 23 pairs
of chromosomes.– 22 of these pairs are identical in both
genders. They are called autosomes.– 1 of these pairs differs between the
genders; this pair is the sex chromosomes (X and Y).• Females possess a homologous pair of X’s
(XX).• Males have one X and one Y (XY; not
homologous).
Sex Chromosomes cont’d• Since females are XX,
all female gametes carry an X chromosome.
• Male gametes may carry either an X or a Y (depending on chance).– So it is the sperm that
determines the gender of the offspring.
Sex Chromosomes cont’d
Sex-Linked Traits• Traits controlled by genes on
the sex chromosomes are called sex-linked traits.–We usually focus on traits
controlled by the X chromosome• This is because it has many more
genes than the Y chromosome!– (Y does code for some traits, too,
but they aren’t as commonly taught.)
Sex-Linked Traits cont’dNotation for sex-linked genes:• Genes carried on the X
chromosome:–Use the letter X to indicate that
the gene is on the X chromosome–Use a superscript letter for the
trait itself.– Eg: XB = dominant gene for
normal colour visionXb = recessive gene for colour-blindness
Sex-Linked TraitsExamples:• Red-green colour-blindness (X-
linked)– XB = normal; Xb = colour-blindGenotype PhenotypeXBXB
XBXb
XbXb
XBYXbY
Genotype PhenotypeXBXB Female; normal visionXBXb Female; normal vision
(carrier)XbXb Female; colour blindXBY Male; normal visionXbY Male; colour blind
Sex-Linked Traits cont’dExamples cont’d:• Hemophilia (deficiency in blood
clotting factors)• Duchenne muscular dystrophy• Some forms of diabetes insipidus
(another form of diabetes – excessive thirst and urination)
• (These are all recessive conditions)
Sex-Linked Traits cont’d• Note: Because males only have
one X chromosome, they cannot be carriers of sex-linked disorders or conditions.– They either have the gene and
show the condition, or don’t have it and don’t show the condition.
Special Cases in Sex Linkage
• X-inactivation:• Females have two copies of the X
chromosome, but males don’t.• The duplicate genes on the second X
chromosome aren’t all necessary! – The body selectively “deactivates” X
chromosomes in some female cells.• Deactivated X’s become inert Barr bodies.
X-Inactivation in Cats• Coat colour genes
are on the X chromosome
• If the cat is heterozygous, then its coat will change from place to place depending on which X chromosome gets inactivated (Calico cat).
Sex-Linked Inheritance Practice Problems
• Both the mother and the father of a colorblind male appear to be normal. From whom did the son inherit the allele for colorblindness? What are the genotypes of the mother, father, and the son?
• A woman is colorblind. What are the chances that her son will be colorblind? If she is married to a man with normal vision, what are the chances that her daughters will be colorblind? Will be carriers?
• Both the husband and the wife have normal vision. The wife gives birth to a colorblind daughter. Is it more likely the father had normal vision or was colorblind? What does this lead you to deduce about the girl’s parentage?
• What is the genotype of a colorblind male with long fingers is s=long fingers? If all his children have normal vision and short fingers, what is the likely genotype of the mother?