Chapter 10.2 Flashcards
Linked genes
Genes found on the same chromosome
Despite genes being on the same chromosome, they are linked physically but…
the allele inherited for one gene does not affect which allele will be inherited for the other gene
Why does the allele for one gene not affect which allele would be inherited for the other gene?
This is because crossing over occurs with enough frequency to randomise the inheritance of the two traits.
Where does the idea that genes are not linked fail?
When loci are far apart on the same chromosome they are inherited as though they are not connected at all. However, some linked genes have loci so close together that inheriting one allele for a gene increases the chance of inheriting whichever version of the other gene is found on the same chromosome
Linked genes frequency
The combination of alleles that were together on the parental chromosomes will always be more common in the offspring. New allele combinations, created by crossing over, are called recombinant and are always less common than parental combinations.
Linkage group
Several genes (two or more) on the same chromosome that do not show independent assortment
Thomas Hunt Morgan’s experiment with sex-linked characteristics
Thomas Hunt Morgan worked with fruit flies (Drosophila melanogaster) to demonstrate that genes are found on chromosomes and that they do not always follow Mendelian ratios. Fruit flies are a useful model organism for studying genetics because they are small and easy to care for, they go through an entire life cycle in about ten days, and they produce hundreds of offspring. The females and males have visible differences, especially in the abdomen. Fruit flies have three pairs of autosomes and one pair of sex chromosomes.
Morgan and his students used a variety of methods, including radiation , to cause heritable mutations with visible effects on phenotypes that could be studied. The first clear exception to Mendelian inheritance found in Drosophila was the inheritance of white eyes. Fruit flies usually have red eyes.
Morgan observed a male fruit fly with white eyes. This unusual mutant male was mated with red eyed females and produced offspring that all had red eyes, demonstrating that red eye colour is dominant. However, the males and females of the F 1 generation produced offspring in F 2 with a ratio of 2 red-eyed females : 1 red-eyed male : 1 white-eyed male. Although overall the ratio of red to white eyes was 3:1, the white-eyed flies were all males. If eye colour had followed Mendelian predictions, an equal number of males and females should have white eyes. Thus, Morgan established sex-linked traits and suggested that some genes, including eye colour in Drosophila , are located on the sex chromosomes.
Thomas Hunt Morgan’s experiment with autosomal characteristics
Many pairs of genes followed expected Mendelian ratios, which assume independent assortment. However, some gene pairs seemed to ‘stick together’. Most offspring had the same combination of alleles as the parents, while far fewer than expected had recombinant traits. These genes show autosomal linkage , meaning they are found on the same autosomal chromosome.
Recombinant
An organism that contains a different combination of alleles to either of its parents.
Tips to remember for linked genes
Whenever non-Mendelian ratios are obtained in a dihybrid cross, the genes are likely to be linked.
The lower the recombinant frequency, the closer the linked genes are on the chromosome.
factors that influence phenotype
polygenic traits - more than one gene may affect a single trait (human eye colour is polygenic because it is determined largely by two genes with some additional influence from genes on at least a dozen loci)
pleiotropic genes - a single gene may affect multiple traits (the CFTR gene codes for a transmembrane protein with effects in multiple organs. When the cystic fibrosis allele is homozygous, the functions of the lungs, intestines and pancreas are all affected.)
epistatic genes - A gene may impact the expression of another gene (the TYR gene produces a polypeptide that is a necessary first step in creating the melanin that gives skin, hair, and eyes their many possible colours. When the TYR gene is not functional, the individual will exhibit albinism no matter which alleles are present in the genes that normally determine the level of pigmentation.)
penetrance of the allele- Some alleles are not expressed in all individuals (BRCA1 alleles have incomplete penetrance because even though they greatly increase the risk of breast cancer, some women with the allele never develop cancer.)
expressivity of the allele - some alleles are expressed with varying intensity (the polydactyly allele in cats (which causes extra toes) has varied expressivity because some cats with the allele have only one or two extra toes, while others have four extra toes per paw.)
Some genetically influenced traits also have important environmental influences (some alleles of the MC1R gene cause freckling when the skin is exposed to sun. The development of freckles has both genetic components and environmental aspects related to the amount and intensity of sunlight the individual experiences.)
Genes and data
A trait with phenotypes that can be grouped into distinct categories shows discrete variation (or discontinuous variation) and is usually coded for by a single gene. An example is the ABO blood group in humans. Traits that show continuous variation display an unbroken range of phenotypes in the population. In other words, for any two phenotypes, there can be an intermediate between them. Traits with continuous variation are often polygenic traits, with the many variations created by combining the effects of various alleles of multiple genes.
Chi-Squared Test equation
x^2 = ∑ (O - E)^2 / E
How to determine the critical chi - squared value
To determine the critical chi-squared value, two other values must first be determined. One is the degrees of freedom. For goodness-of-fit, that will be one less than the total number of categories. In this example, there are two possible outcomes: yellow seeds and green seeds. That means the number of degrees of freedom is (2–1)=1 degree of freedom. The second value is the significance level, which has a range of 0–1. The p -value, also between 0–1, is the derived probability that the chi-squared value would be as large or larger than the calculated value if the null hypothesis were true. In other words, it is the probability that the difference observed (or a greater difference) would occur randomly in a situation where there is no meaningful difference. The significance level is usually set at 0.05, which is equivalent to a 5% probability that the difference between observed and expected results would occur by chance assuming the null hypothesis is correct
