01:447:380 Final: Genetics Final
Sex Determination & Sex-Linked Traits and Disorders
Important Terms
• Monoecious = “one house,” aka hermaphroditic—one organism possesses both sex’s organs
• Dioecious = “two houses,”—each organism possesses one sex’s organs
• Autosomes = Numbered chromosomes; sex chromosomes = X and Y in many organisms, Z and W
in others (like birds)
• Heterogametic = Produce two different types of gametes (i.e. X-bearing vs Y-bearing sperm)
• Homogametic = All gametes have the same chromosome content (females)
Several Mechanisms For Sex Determination
MOST DIOECIOUS ORGANISMS USE A CHROMOSOMAL MECHANISM FOR SEX DETERMINATION
• XX-XO System In Grasshoppers
o Females have two Xs, males have one X—there is no “O” chromosome
o Males are heterogametic—X-bearing sperm versus sperm with no sex chromosome
(represented by using the “O” in the Punnett square); both has set of autosomes
• Haplodiploidy In Bees, Wasps And Ants
o Bees do not have sex chromosomes--Female bees are diploid; males are haploid
o The queen stores the male’s sperm in a separate compartment, and fertilizes only some of
her eggs—environmental and other factors influence the percent of eggs fertilized
*Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are
haploid, and females develop from fertilized eggs and are diploid.
• XX-XY System In Some Plants, Some Insects, Some Reptiles And All Mammals
o Females have XX, males XY
o Males are heterogametic--X-bearing sperm versus Y-bearing sperm
o While these are called “chromosomal mechanisms,” keep in mind that it is the genes that
are on these chromosomes that determine sex
• ZZ-ZW system in birds, snakes, butterflies, some amphibians and some fish
o Females have ZW, males have ZZ
o Females are heterogametic—Z-bearing eggs versus W-bearing eggs
XX Males And XY Females Illustrate The Importance Of The SRY Gene
• The X and Y chromosomes only recombine like autosomes do at the ends—called the
pseudoautosomal regions
*The pseudoautosomal regions (PAR1 and PAR2) of the
human X and Y chromosomes pair and recombine during
meiosis.
*Genetic recombination is the production of offspring with
combinations of traits that differ from those found in either
parent.
• The sex determining region of the Y (SRY), aka testis
determining factor (TDF), lies just below the
pseudoautosomal region on the p arm of the Y
chromosome
• If the tip ends of the X and Y exchange more material
than they are supposed to, the SRY gene can get
translocated onto the X chromosome
This happens during spermatogenesis; the mother will provide
an X chromosome in the egg
• If the sperm that makes the child contains the X that has the SRY gene
on it, the child will develop as a male, despite having two X
chromosomes
• If the sperm that makes the child contains the Y that has lost the SRY
gene, the child will develop as a female, despite having a Y
chromosome
Imbalance of Sex Chromosomes Turner Syndrome: 45, X
• Female (because they have one X)
o Do not go through puberty normally (this patient is 16 years old)
o Some have mild cognitive deficits
o Many have webbed necks
o Chubby body type
Imbalance Of Sex Chromosomes Klinefelter Syndrome: 47, XXY
• Male (because they have a Y)
o Secondary sexual development is somewhat female
o Gynecomastia (male breasts)
o Very low sex drive
o Sterile
Fruitflies Use A Genic Balance System
• Fruit-flies have X and Y chromosomes, but the Y chromosome does not help determine sex—some
females have a Y chromosome
• The Y chromosome acts like a sex chromosome during meiosis, in that it and the X will segregate
apart from each other into different gametes
• In Drosophila, sex is determined by the balance between the number of copies of certain X-linked
genes versus certain autosomal genes, i.e.
the ratio between the number of X
chromosomes and the number of haploid
sets of autosomes (not total number of
autosomes)
• Many Possibilities For The X:A Ratio
o Most flies have two sets of
autosomes; some have three
o A fly may have up to four X
chromosomes
Fruit flies Use a Genic Balance System
(X:A ratio)
Sex
0 – 0.5
Metamale
Male phenotype, low viability/fertility
0.5
Male
1 x and 2 sets of automsomes
0.5 – 1.0
Intersex
Both male and female traits
1.0
Female
2 x and 2 sets of automsomes
>1.0
Metafemale
Female phenotype, low viability/fertility
Haplodiploidy in Bees, Wasps and Ants
• Unfertilized eggs develop into haploid males whose genomes are derived solely and completely
from their mother (one of the mom’s allele for each gene)
• For every gene, the son has one of the two alleles the mother possesses
• Fertilized eggs develop into diploid females, who have half their genes from their mother and half
from their father.
*Females produce haploid eggs by meiosis, males produce haploid sperm by mitosis
In Some Organisms, Environmental Factors Determine Sex
• In turtles, crocodiles and alligators, temperature determines sex
o In turtles, warm temperatures produce females, cooler temperatures produce males
o In alligators, the reverse is true
Inheritance of Sex-Linked Traits Demonstrating X-Linked Inheritance In Fruit flies
• The American biologist Thomas Hunt Morgan first introduced the concept of sex-linked
inheritance approx. 1915
• Mendel had presented his paper in 1865; it was well-received, but people paid more attention to
the implications for hybridizing plants than to the principles of inheritance
• Around 1900, several people rediscovered Mendel’s work, and used it as the foundation for what
became the accepted model for all inheritance
Mendel’s principles apply well to traits that are determined by autosomal genes, but not to sex
chromosome-linked genes
• At first an embryologist, Morgan got interested in genetic research, and while performing research
using fruit flies, he noticed a white-eyed male (fruit flies have red eyes)
• He crossed the white-eyed male with a pure-breeding red-eyed female and saw that all the F1s
had red eyes
This is consistent with what everyone assumed about the principles of inheritance at that time--that this
cross illustrated that the red allele was dominant over the white allele, and the F1s were heterozygous
for the red/white alleles
• When he crossed the F1s, however, he found that, in the F2 generation, all females had red eyes,
and half the males had red eyes, while half the males had white eyes
• Under the Mendelian assumptions that apply to the autosomes, you would expect both the male
and female offspring of two heterozygous parents
(i.e. Red/White X Red/White) to show the following genotypes and phenotypes:
¼ Red Red
Red Eyes
½ Red White
Red Eyes
¼ White White
White Eyes
• Morgan hypothesized that the eye color gene is on the X
chromosome, and that females had two Xs, where males had
one
Morgan’s First Demonstration of X-Linked Inheritance in Fruit
flies
X gene --> eye color, X+ --> red (wild-type, dominant), XW --> white
(recessive)
Document Summary
Zz-zw system in birds, snakes, butterflies, some amphibians and some fish: females have zw, males have zz, females are heterogametic z-bearing eggs versus w-bearing eggs. Xx males and xy females illustrate the importance of the sry gene: the x and y chromosomes only recombine like autosomes do at the ends called the pseudoautosomal regions. *the pseudoautosomal regions (par1 and par2) of the human x and y chromosomes pair and recombine during meiosis. If the tip ends of the x and y exchange more material than they are supposed to, the sry gene can get translocated onto the x chromosome. This happens during spermatogenesis; the mother will provide an x chromosome in the egg. If the sperm that makes the child contains the x that has the sry gene on it, the child will develop as a male, despite having two x chromosomes.
