Genetics 2 Flashcards
(15 cards)
Chromosomal basis of Mendel’s laws
Starting with two true-breeding pea plants, we will follow two genes through the F1 and
F2 generations.
The two genes specify seed color (allele Y for yellow and allele y for green) and seed shape (allele R
for round and allele r for wrinkled).
• These two genes are on different chromosome pair
Extending Mendelian Genetics for a single gene
• Inheritance of characters by a single gene may deviate from simple
Mendelian patterns in the following situations
A)When alleles are not completely dominant or recessive
B) When a gene has more than two alleles
C) When a gene produces multiple phenotypes.
Degrees of dominance
When alleles are not completely dominant or recessive
Degrees of Dominance
• Complete dominance
• Incomplete dominance
• Codominance
• Overdominance
Degrees of Dominance
• Complete dominance occurs when phenotypes of the
heterozygote and dominant homozygote are identical.
• In Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
• In Incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
• Alleles can show different degrees of dominance and recessiveness in relation to each other.
• In Mendel’s classic pea crosses, the F2 offspring always looked like one of the two parental varieties because one allele in a pair showed complete dominance over the other.
• Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical.
• In such situations, the phenotypes of the heterozygote and the dominant homozygote are indistinguishable
Codominance
• Another variation on dominance relationships between alleles is called codominance.
• In this variation the two alleles each affect the phenotype in separate, distinguishable ways
E.g. 1 white flowered plant + 1 red flowered plant = Red and white flowered plant
Incomplete dominance
• For some genes, neither allele is completely dominant and the F1 hybrids have a phenotype somewhere between those of the two parental varieties.
• This phenomenon called incomplete dominance is seen when red snapdragons are crossed with white snapdragons: all the F1 hybrids have pink flowers.
• When red snapdragons are crossed with white ones, the F1 hybrids have pink flowers.
• Segregation of alleles into gametes of the F1 plants results in an F2 generation with a 1:2:1 ratio for both genotype and phenotype.
• Neither allele is dominant, so rather than using upper- and lowercase letters, we use the letter “C” with a superscript to indicate an allele for flower colour CR for red and for CW white.
Multiple alleles
• Most genes exist in populations in more than two allelic forms.
• For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells.
• The alleles are IA, IB, and i.
• A person’s blood group may be one of four types: A, B, AB, or O. These letters refer to two carbohydrates, A and B, that may be found attached to specific cell surface molecules on red blood cell
The three alleles for the ABO blood groups and their carbohydrates.
Each allele codes for an enzyme that may add a specific carbohydrate (designated by the superscript on the allele and shown as a triangle or circle) to red blood cell
• The enzyme encoded by the IA allele adds the A carbohydrate.
• The enzyme encoded by the IB allele adds the B carbohydrate.
• The enzyme encoded by the i allele adds neither.
• Matching compatible blood groups is critical for safe blood transfusions
Multiple alleles for the ABO blood group
• ii individuals are recessive homozygotes
• Do not express A or B
• IA and IB are codominant
• Both are expressed in the IAIB
heterozygote phenotype
Overdominance and the Inheritance of Sickle Cell Disease
• Overdominance is the phenomenon in which a heterozygote is more vigorous than both of the corresponding homozygotes
• e.g. Sickle-cell anaemia
• Carriers of the HbS allele have partial resistance to malaria
HbAHbS individuals have an “advantage” over
• HbSHbS, because they do not suffer from sickle cell anemia
• HbAHbA because they are more resistant to malaria
Pleitropy
When a gene produces multiple phenotypes.
➢ One gene = one phenotypic character BUT most genes have multiple phenotypic effects
➢ Ability of a gene to effect an organism in multiple ways is pleiotropy
➢ the gene that determines flower color also affects the color of the coating on the outer surface of the seed, which can be gray or white
➢ In humans, for example:
➢ pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
Epistasis
• In Epistasis, the phenotypic expression of a gene at one locus alters that of a gene at a second locus.
• For example, in rats and many other mammals, coat colour depends on two genes.
• One gene determines the pigment colour (with alleles B for black and b for brown).
• The other gene (with alleles C for colour and c for no colour) determines whether the pigment will be deposited in the hair
Epistasis is a circumstance where the expression of one gene is modified (e.g., masked,
inhibited or suppressed) by the expression of one or more other gene
Polygenic Inheritance
• Mendel studied characters that could be classified on an either-or basis, such as purple versus white flower color.
• But many characters, such as human skin color and height, are not one of two discrete characters, but instead vary in the population in gradations along a continuum.
• These are called quantitative characters.
• Usually quantitative variation indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypic character
A simplified model for polygenic inheritance
• In this model, three separately inherited genes affect skin color.
• The heterozygous individuals (AaBbCc) represented by the two rectangles at the top of this figure each carry three dark-skin alleles (black circles, which represent A, B, or C) and three light- skin alleles (white circles, which represent a, b, or c)
• The Punnet square shows all the possible genetic combinations in gametes and offspring of many hypothetical matings between these heterozygotes.
• The results are summarized by the phenotypic frequencies (fractions) under the Punnet square
Nature vs nurture
• An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
• An organism’s phenotype reflects its overall genotype and unique environmental history
“The product of a genotype is not a rigidly defined phenotype but a range of phenotypic possibilities with variation due to environmental influence”
Phenotype range is called “norm of reaction”
The Environment impact of phenotype
For example hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
Another example is the snow hare. In the summer the snow hare has brown fur, but in winter grows white fur as an adaptation for camouflage
Nature and Nurture:
The Environmental Impact on Phenotype
• Norms of reaction are generally broadest for polygenic characters.
• Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype.
• For humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests.
• Even identical twins, who are genetic equals, accumulate phenotypic differences as a result of their unique experiences.
• Whether human characters are more influenced by genes or the environment «nature vs nurture», is a long standing debate that we will not attempt to settle here.
• Usually a genotype is not associated with a rigidly defined phenotype, but rather with a range of phenotypic possibilities due to environmental influence
