Genetics: Sex linkage Flashcards
Test review
Why were fruit flies used to test genetic studies on sex linkage?
1: They produce many offspring
2: They have a short reproductive cycle (14 days) so many generations in short time
3: Large numbers can be kept in small places
4: They only have 4 chromosome pairs
How was sex determination discovered?
Scientists discovered males and females had one chromosome pair different. Females had 2 (rod shaped) X chromosomes, males had 1 X shaped and 1 (cane) Y shaped chromosome. These chromosomes were named sex chromosomes due to their relation to gender. All other chromosome pairs are autosomes.
How many pairs of chromosomes do humans have?
23 chromosome pairs
22 of which are autosome pairs
1 of which are sex chromosomes (XX = female and XY = male)
Who discovered the chromosome theory of inheritance?
Walter Sutton and Theodor Boveri.
What is the chromosome theory of inheritance?
Theory explaining that genes are carried on chromosomes. Explains cellular basis of Mendel’s work and his segregation and independent assortment laws (Movement of homologous pairs during meiosis). Proposed the question of if each gene has its own chromosome.
Why do genes occur in pairs?
Because they are carried on homologous chromosomes, one from each parent.
What are linkage groups?
Sutton’s fruit fly observations revealed each chromosome carried many genes. Genes on the same chromosome are in the same linkage group. He suggested that these genes don’t change positions, and will be inherited together.
What was T. H. Morgan’s experiment on sex linkage?
Morgan realized that some fruit flies had white eyes not red, and all white eyed were males. He crossed white eyed males with red eyed females (P), and all male and female offspring (F1) had red eyes, meaning white eyes were recessive. He then crossed F1 flies and got a PR of 3 red:1 white, with all white eyes being males. Morgan concluded eye colour gene was located on the X chromosome, but the Y chromosome did not carry a gene for it.
What are examples of sex-linked traits?
1: Hemophilia in humans
2: Red-green colour blindness in humans
3: White eyes in fruit flies
What are the possible genotypes for red-green colour vision?
X^R X^R - Woman with normal vision
X^R X^r - Woman with normal vision, carrying colour blind allele
X^r X^r - Colorblind woman
X^R Y - Man with normal vision
X^r Y - Colorblind man
What is Gene Linkage/Gene Chromosome Theory?
Many genes are located linearly along a chromosome. All genes located on the same chromosome belong to the same linkage group. Linked genes do not follow Mendel’s independent assortment laws, since they are inherited together.
What is the relation between crossing over and incomplete linkage?
Linked genes sometimes behave like unlinked genes during crosses. Occurs due to genes breaking apart (incomplete linkage) due to crossing over of homologous chromosomes in prophase 1.
What is chromosome mapping?
The further apart that genes are located from each other on a chromosome, the more likely they are to break apart. Analyzing the frequency of certain genes being crossed over, scientists can figure out the relative position of genes on chromosomes.
