Sex Linkage (Genetics) Flashcards
Who is the famous scientist for determining sex linkages in the 1900s?
Thomas Morgan
What organisms were used in experiments for sex linkage?
Fruit flies
What is the wild type allele for the fruit fly?
red eyes
What is the mutant allele for the fruit fly?
white eyes
T or F: the mutant allele was found in both female and male offspring
FALSE. Only F2 generation MALES
What phenotype did the F1 offspring have when a female fruit fly with red eyes was crossed with a male fruit fly with white eyes?
All had red eyes
When the F1 of the fruit flies were crossed, what phenotypes were present? and in which ratio?
3 red eyes: 1 white eye but the only ones with white eyes were the males
Which allele was dominant and which was recessive in Morgan’s experiments?
Red eyes dominant
White eyes recessive
Define sex chromosome
chromosomes involved in determining if an organism is male or female (X or Y)
In humans and fruit flies, what is the typical genotype for biological males?
XY
In humans and fruit flies, what is the typical genotype for biological females?
XX
Where did Morgan determine the gene for eye colour is? Why did he decide this?
On the X chromosome because males have only ONE X chromosome so they have to display whichever genes they get on the X.
How come the females didn’t show the white eyed phenotype?
Because females have two X chromosomes, so they would only show the white eyes if they had two white-eye alleles (one on each X)
So females will show the red eye phenotype if they are hetero- or homozygous for red
Define reciprocal crosses
A pair of matings where the phenotypes of the males and females are different (parents are true bred)
What are reciprocal crosses used for? What do the results show?
Testing for sex linkage
If the results are different, the condition is SEX LINKED
if the results are the same, it’s autosomal
Which sex chromosome is much shorter than the other?
the Y is much shorter than the X
During meiosis in the testes and ovaries, what occurs to the X and Y chromosomes?
They segregate
Which chromosome(s) can females produce in gametes?
Only the X
Which chromosome(s) can males produce in gametes?
50% X and 50% Y
What biological sex will result if a sperm containing a Y chromosome fertilizes an egg?
A male offspring
What biological sex will result if a sperm containing an X chromosome fertilizes an egg?
A female offspring
How do alternatives of sex chromosome numbers and/or pairs occur?
Due to incorrect separation of chromosomes during meiosis
Describe the syndrome related to having only an X chromosome
Turner Syndrome
Occurs in females, usually shorter in height and infertile
Describe the syndrome related to having XXX chromosomes
Trisomy X
2 Xs are inactivated so mild or no symptoms occur but can be taller
Describe the syndrome related to having only a Y chromosome
Not viable, organisms need an X to survive
Describe the syndrome related to having XXY chromosomes
Klinefelter Syndrom
Usually lower in testosterone, fertility is usually impacted
Describe the syndrome related to having XYY chromosomes
XYY Syndrome
Usually taller, no symptoms, and testosterone is normal
When do sexual differences begin to occur in human embryos?
around 2 months
Which sex chromosome determines sex? How?
Y
A gene on the Y chromosome is responsible for the development of testes (SRY)
If there is no SRY present, the embryo will develop ovaries
Define a sex-linked gene
A gene located on an X or Y chromosome
About how many genes does a Y chromosome have?
~78
About how many genes does a X chromosome have?
~1100
T or F: There are more X-linked disorders than Y-linked?
TRUE
T or F: Fathers pass their X-linked alleles to their daughters, but not their sons
TRUE
T or F: Mothers pass their X-linked alleles to their sons, but not their daughters
FALSE. Mothers pass their X-linked alleles to BOTH daughters and sons
When will a female offspring express a recessive X linked trait?
Only if they are homozygous for that trait
Will a female express a recessive X linked trait if they are heterozygous for that trait?
Unlikely
Define carrier
An organism that often doesn’t display the phenotype of the recessive trait but can transmit that allele to their offspring
Can males be carriers?
NO because they only have one copy
What are the symbols for the genotype of an unaffected female?
XNXN
N is superscript
What are the symbols for the genotype of a phenotypically unaffected but carrier female?
XNXn
N and n is superscript
What are the symbols for the genotype of an affected female?
XnXn
n is superscript
What are the symbols for the genotype of an unaffected male?
XNY
N is superscript
What are the symbols for the genotype of an affected male?
XnY
n is superscript
How are the results of a Punnet Square reported?
Unaffected females: carrier females: unaffected males: affected males
X is always listed before Y
Give 3 examples of X-linked disorders
- Colour blindness (recessive)
- Muscular dystrophy (recessive)
- Hemophilia (recessive)
Define Barr Body
Occurs in females when 1 of the 2 X chromosomes is randomly chosen to be inactivated during embryonic development
Where is the Barr Body located?
Inside the nuclear envelope
T or F: most of the genes on the Barr Body are not expressed
True
When are the Barr Body’s reactivated?
In female gametes
Give an example of a heterozygous female for an X-linked trait with half the cells expressing each allele
Calico cats
Define pedigree
A family tree with the history of a particular trait
What is the purpose of a pedigree?
A way to study human genetics and lineage by analyzing matings that have already occurred to look at the transmission of certain traits
When examining a pedigree, how can you tell if a condition is sex linked or autosomal?
If only the male offspring are affected it is sex linked
If both male and female offspring are affected equally, it is autosomal
