17.7 - Autosomal Linkage Flashcards
What is autosomal linkage?
- Autosomal linkage occurs when two or more genes are carried on the same autosome (one of the 22 non-sex chromosomes).
- Genes located on the same chromosome do not segregate independently during meiosis, and are inherited together unless crossing over occurs.
What is a linkage group?
- A linkage group consists of all the genes on a single chromosome.
- Genes on the same chromosome are said to be linked and are typically inherited together, forming a linkage group.
What is the difference between autosomes and sex chromosomes?
- Autosomes are the 22 pairs of chromosomes that do not determine sex, while the 23rd pair are the sex chromosomes (X and Y in humans).
- Genes on autosomes are involved in autosomal linkage, while genes on sex chromosomes are sex-linked.
How does autosomal linkage affect Mendel’s Law of Independent Assortment?
- Autosomal linkage means that linked genes do not segregate independently, which contradicts Mendel’s Law of Independent Assortment.
- Linked genes are inherited together during meiosis unless crossing over occurs, reducing the variety of gametes produced.
How does crossing over affect autosomal linkage?
- If crossing over occurs during meiosis, it can separate linked genes, allowing them to assort independently.
- However, if there is no crossing over, linked genes remain together and are passed on as a unit to offspring.
What are the possible gamete combinations if two linked genes do not undergo crossing over?
- If two linked genes do not undergo crossing over, only two possible gamete combinations are produced: one containing both dominant alleles (AB) and one containing both recessive alleles (ab), rather than the four combinations (AB, Ab, aB, ab) expected if the genes were unlinked.
Provide an example of autosomal linkage in Drosophila melanogaster.
- In Drosophila melanogaster, one gene controls body color (grey dominant to black), and another gene controls wing size (normal wings dominant to vestigial wings).
- If these two genes are linked, a heterozygous fly (GgNn) would produce only two types of gametes (GN, gn) instead of the four possible combinations if the genes were on separate chromosomes.
What offspring ratio is expected in a dihybrid cross if the genes are not linked?
- In a dihybrid cross with independent assortment (no linkage), the expected phenotypic ratio in the offspring is 9:3:3:1.
- For example, in Drosophila, this would result in:
- 9 grey body and normal wings
- 3 grey body and vestigial wings
- 3 black body and normal wings
- 1 black body and vestigial wings.
How does autosomal linkage alter the expected offspring ratio in a dihybrid cross?
- When two genes are linked on the same autosome and there is no crossing over, fewer recombinant offspring are produced, and the 9:3:3:1 ratio seen with independent assortment does not apply.
- Instead, offspring primarily inherit the parental combinations of traits (GN, gn).
Do genes assort independently even when they are linked
- no
- linked genes do not follow Mendel’s Law of Independent Assortment unless crossing over occurs.
A heterozygous fruit fly (GgNn) with linked genes for body color (G/g) and wing size (N/n) produces offspring. What gametes are possible if there is no crossing over?
- If there is no crossing over, the heterozygous fruit fly (GgNn) will only produce two types of gametes: GN and gn.
- Students often miss this and predict four types of gametes (GN, Gn, gN, gn) assuming independent assortment, which is incorrect when genes are linked.
How can you predict if two genes are linked in an exam question?
- If two genes show fewer recombinants than expected or produce offspring with parental traits more frequently than the 9:3:3:1 ratio, it suggests the genes are linked.
- When analyzing genetic crosses, if the expected ratios don’t match the Mendelian ratio, linkage may be involved.
Why is crossing over important for genetic variation, especially in linked genes?
- Crossing over during meiosis is essential for creating genetic diversity, particularly in linked genes.
- It allows for the exchange of genetic material between homologous chromosomes, breaking up linked genes and resulting in recombinant gametes with new allele combinations.
Can autosomal linkage be traced using pedigree charts?
- Yes, autosomal linkage can be analyzed using pedigree charts by observing the inheritance patterns of traits across generations.
- If two traits consistently appear together in offspring, it suggests that the genes controlling these traits are linked.
Avariety of rabbit has an allele (H) for long hairs and another allele (h) short hairs. It also has an allele (G) for grey hairs and another allele (g) for white hairs. The gene for hair length and the gene for hair colour are on the same chromosome,
Draw a genetic diagram to show the results of a cross between a rabbit that has short, white fur and one that is heterozygous for both characters.
