Ch. 3 Extension of Mendelian Genetics Flashcards
Was Mendel Wrong?
After the rediscovery of Mendel’s Laws of inheritance in the early 1900s, scientists and agriculturists tried to repeat Mendel’s experiments.
Many confirmed Mendel’s finding, but many others did not
Campions petal color and foliage cross showed what?
Did not show a 9:3:3:1 distribution in the F2 generation as Mendel has expected
What test is used to compare the observed and expected values?
Chi-square test.
Is deviation due to change or not?
Chi- square formula
x^2= the sum of (observed - expected)^2/expected
Degrees of freedom
n-1 (group size-1)
How do you compare chi square to critical value?
If chi-square is smaller than the critical value, fail to reject the hypothesis. Means that deviation was due to chance.
If chi-square is larger than the critical value, reject the hypothesis. Deviation is not due to chance.
Basic principles of gene transmission (Mendel’s Laws)
Genes are present on homologous chromosomes.
Chromosomes segregate and assort independently during meiosis and the formation of gametes.
Alleles are transmitted from parent to offspring following Mendelian rules.
Alleles can change phenotypes (what Mendel didn’t know)
Alleles often do not display a dominant/recessive relationship.
Phenotypic ratios are not always 3:1 or 9:3:3:1 as expected following Mendel
What are alleles?
Different versions of a gene.
Impact how a gene is expressed.
If the gene is expressed (translated) into a protein, an alternative allele might be expressed into a different gene product (a different protein).
Different proteins (or the lack of) impact the phenotype differently.
What is the relationship between genes and protiens?
Gene -> protein -> phenotype
Garden pea plant Sbe1 gene (starch branching enzyme)
sbe1 enzyme- catalyzes the formation of highly branched starch molecules during seed maturation.
sbe1 gene has 2 alleles:
R- codes for functional Sbe1 protein
r- codes for non functional sbe1 protein
Garden pea
R and r alleles - nucleotides
R - represents 3500 nucleotides
r - represents 3500 nucleotides plus an additional 800 nucleotides that make the gene non functional
Garden pea
Wrinkled gene
Wild type (R) encodes for Sbe1 - catalyzes the formation of highly branched starch
Mutant (r) is not coding for this enzyme, hence wrinkled seeds lack enzyme activity.
No enzyme activity -> more sucrose -> water moves in by osmosis, causing the pea to expand inside its seed coat. Once seed matures it loses the water as it dries and shrivels. But having been stretched, the seed coat then wrinkles as the pea inside shrinks -> wrinkled appearance.
Wild type allele
The allele which normally occurs in a wild population, most frequently, often (but not always) dominant. Responsible for the wild type phenotype.
Mutant allele
Alternative alleles resulting in altered gene product.
Arises through mutations.
Responsible for the mutant phenotype.
What are the types of mutant alleles?
Loss of function mutation
Gain of function mutation
Neutral mutations
Loss of function mutation
(hypomorphic, amorphic) gene is coding for an enzyme, mutation causes reduction or elimination of that enzyme (null allele) wrinkled gene is an example
Metabolic disorders
Gain of function mutation
(hypermorphic) mutation enhances function of wild type, excess gene product (ex: conversion of proto-oncogenes which regulate cell cycle to oncogenes where regulation is overridden by excess gene product -> cancerous cell)
Neutral mutation
No change to the phenotype, no change to the evolutionary fitness of the organism (use in phylogenetics and population genetics- can see mutations in sequence of DNA)
Genes function to produce polypeptides
Most genes are translated into polypeptide (amino acid chains); alteration of genes causes alteration in the polypeptide chains.
(Mutations in a gene cause alterations in polypeptide chain and that can influence the expression in the phenotype)
Wild type allele produces what type of polypepetide?
A functional polypeptide.
Wild-type phenotype
Recessive amorphic loss of function allele produces?
Does not produce a functional polypeptide.
Server mutant phenotype.
Recessive allele
Recessive hypomorphic loss of function allele produces?
A partially functional polypeptide.
Mild mutant phenotype.
Recessive allele
Dominant negative allele produces?
A polypeptide that interferes with the wild-type polypeptide.
Severe mutant phenotype.
Dominant allele
Amorphic
protein is completely different and non functional
Hypomorphic
protein is slightly different, still functional, but less active
Wild type and amorphic genotype produces?
Wild type phenotype
Mutant allele is recessive
Wild type and hypomorphic genotype produces?
Wild type phenotype
Mutant allele is recessive
Wild type and dominant genotype produces?
Mutant phenotype
Mutant allele is dominant (mutant gene blocks the wild type one)
Dominant allele symbols
indicated by italic uppercase letter (D) or letters (Wr)
Recessive allele symbols
indicated by either an italic lowercase letter (d) or italic letter or group of letters (wr)
Mutant alleles symbol
indicated by italic letter (e)
Wild type allele symbol
indicated by italic letter plus superscript + (e^+)
Nutritional mutants in bacteria
Which amino acid they can or can’t synthesize
leu- refers to mutant which cannot synthesize amino acid leucine, the wild type bacteria would be leu+
Mutants in humans
capital, italicized letters are used to name genes: BRAC1, one of the genes associated with susceptibility to breast and other cancers
What are the types of dominance?
Complete dominance
Incomplete dominance
Codominance
Dominance does not alter the way the genes are inherited, it only influences what?
Influences the way the genes are expressed as a phenotype
Incomplete or partial dominance
The phenotype of the heterozygote is midway between the phenotypes of the 2 homozygotes.
One allele is partially, or incompletely, dominant over the other.
Is incomplete dominance the same as the blending hypothesis?
NO. When you cross 2 heterozygous (pink) plants, the homozygous phenotypes (red and white) reappear. If the blending hypothesis was true, pink parents would cause offspring to be even lighter and the alleles would get watered down through generations.
Tay-Sachs Disease - what is it?
Rare recessive autosomal disease.
2 unaffected heterozygous parents (Aa) have a probability of 1/4 of having a child with TSD (aa).
Infants unaffected at birth until 6 months, progressive loss of mental and physical state, leads to death usually by 2-3 yrs.
Tay-Sachs Disease - Incomplete dominance
Responsible gene, encodes for alpha subunit of the Hex-A enzyme (hexosaminidase).
Hex-A in lysosomes breaks down a lipid component (ganglioside, GM2) of nerve cell membranes.
Without functional Hex-A, GM2 accumulates within neurons in bran (loss of function mutation).
TSD results from loss of activity of enzyme Hex-A.
Heterozygous carriers (Aa) produce 50% of normal amount of Hex-A - intermediate level of enzyme
Carriers (Aa) show no symptoms of the disorder
Codominance
Neither allele is dominant over the other.
Heterozygous expresses the phenotypes of both homozygotes.
Ex: Coat color in longhorn - appears blended but actually red and white independent hairs
Complete dominance
phenotype of the heterozygote is the same as the phenotype of one of the homozygotes
What if there are more than 2 alleles?
An individual can only have 2 alleles at a distinct gene locus (bc only 2 chromosomes), but multiple alleles can exist within a population at a distinct gene locus.
EX: blood types
Multiple alleles # of genotypes formula
Number of genotypes = [n(n+1)/2]
n= number of alleles