Sex Determination and Linkage Flashcards
What is the chromosomal theory of inheritance? How did it develop? What are the three main parts which comprise this theory?
The Chromosomal Theory of Inheritance:
1) Genes are located on chromosomes
2) The chromosomes undergo segregation and independent assortment during meiosis
3) Meiosis and fertilization account for inheritance patterns
The theory was developed by combining Mendel’s hypothetical “hereditary factors” with modern advances in cell biology in the 1870-90s (the discoveries of mitosis and meiosis)
What contributions to genetic were made by Thomas Hunt Morgan? What model organism did he use? Why are drosophilia a good model to study genetics?
Morgan determined that specific genes were located on specific chromosomes, and that genes on sex chromosomes show unique inheritance patterns
He did this by studying fruit flies (drosophilia)- they were good because they produce many offspring, breed every two weeks, and only have four chromosomes
What are wild type and mutant alleles? What unique event did he observe after doing monohybrid crosses for fly eye color?
Wild type- normal, most common
Mutant- not wild type
He observed after doing a red-eyed female x white-eyed male cross that, while the ratio was still 3:1 in favor of red eyes, only male flies inherited white eyes
What is meant by heterogametic and homogametic sex? Which sex chromosome governs sex determination? What is the name of the gene locus which serves as a switch during embryonic development?
Heterogametic- sex determinate chromosomes are not the same, ie. XY
Homozygous- sex determinate chromosomes are the same, ie. XX
In the case of humans, the Y chromosome governs sex determination- the SRY region on the Y chromosome is the genetic “switch” that initiates testes development, in the lack of this chromosome, the “default” sex is female
How can a woman with XY chromosomes or a man with XX chromosomes be born?
Meiotic recombination occurs b/w the X and Y chromosomes. While it is rare, the SRY region is sometimes swapped onto the X chromosome, leading to people who are biologically male with XX, and people who are biologically female with XY
These individuals are often infertile/sterile, however
How is sex determined in the following non-human species:
birds
grasshoppers
bees
Birds- have Z and W sex-determinate chromosomes; ZW = female, ZZ = male
Grasshopers- have the X-0 system; XX = female, X= male
Bees- have the haplo-diploid system; females are diploid, males are haploid
Why is inheritance of alleles unique to the X chromosme? What is meant by the term hemizygous? Why are men more likely than women to have sex-linked recessive disorders?
While most genes on the Y chromosome are purely sex-related, the X chromosome has many genes necessary for life, and has specific patterns of inheritance, in order for a recessive trait to be present:
A female needs two copies of the allele
A male only needs one copy (hemizygous) b/c there is no other X allele to counteract it
Thus, males are more likely to inherit X-linked recessive disorders
What is hemophilia? What is meant by the term “carrier”? Can women get X-linked diseases?
Hemophilia- an X-linked recessive disorder associated with inactive or ineffective blood clotting enzymes; individuals with hemophilia are susceptible to heavy bleeding from any break in circulation, however minor.
Carrier- a woman carrying a single copy of the allele for a recessive disease, are largely unaffected by the disease
Women can get X-linked diseases if the father has the disease and the mother is at least a carrier
