L12 - Heredity
Genetics – Current News
Many resources and programs related to genetics in the USA are being cut unpredictably and suddenly.
These cuts have negative effects on global health and progress in genetics research.
Lecture 11 – Basic Principles of Heredity (Mendelian Genetics)
Learning Objectives
Explain key principles of heredity initially discovered by Gregor Mendel, including:
Genetic information is passed from parents to offspring via genes.
The two alleles at each locus in the parent separate randomly during gamete formation (segregation).
Genetic information from parents combines in offspring.
Some alleles are 'dominant' or 'recessive' to others (concept of dominance).
Alleles at different loci are transmitted independently if they are on different chromosomes or unlinked on the same chromosome; this involves the random separation of homologous chromosomes, addressed using dihybrid crosses.
Use principles of heredity to predict the proportion of progeny with a given genotype and phenotype for a particular genetic cross in two different ways:
Using Punnett squares
Applying probabilities
Make predictions for:
a. Monohybrid crosses
b. Dihybrid crosses
Mendel's Findings
Monohybrid Crosses
Offspring receive genetic information from both parents.
Principle of Segregation (alleles at one locus):
An individual has two separate alleles at a given locus.
These alleles separate during gamete formation.
They separate in equal proportions.
Principle of Independent Assortment (genes at different loci):
Alleles at different loci separate independently if unlinked.
Crossing over can occur during anaphase II.
One allele can be ‘dominant’ to another.
Dominance and Recessivity
Example 1: Recessive Mutation
Wild type allele (+) is dominant to mutant allele (m).
No functional protein is produced by the mutant allele.
Sufficient protein is produced by the wild type allele.
Mutation displays a recessive pattern of inheritance where only individuals with two 'm' alleles exhibit the disease (e.g., Phenylketonuria - PKU).
Definition of haplosufficient: An allele that can produce enough functional protein with just one copy.
Example 2: Dominant Mutation
This mutation shows a dominant pattern of inheritance where individuals with a single 'M' allele exhibit the disease.
Mechanisms of dominance can include:
Haploinsufficiency: A single wild type (+) allele does not produce enough protein.
Dominant Negative Mutation: Mutant polypeptides distort protein function and can interfere with the function of normal proteins.
Cellular Mechanisms in Mendelian Traits
SBEI Gene: Highly expressed in chloroplasts/amyloplasts during early seed development and catalyzes a reaction in the starch biosynthesis pathway (Smith, 1988; Edwards et al., 1988).
The wild-type round seed allele is dominant over the wrinkled allele due to genetic variation influencing phenotype.
A transposable element insertion in the wild-type starch branching enzyme SBEI allele increases starch and lipid content, which leads to a wrinkled phenotype due to water entering the seed through osmosis (Bhattacharyya et al., 1990; 1993).
Predicting Outcomes of Genetic Crosses
Methods
Punnett Squares
Probability
Key principle of segregation: Two alleles at a locus segregate independently and with equal probability into gametes.
Must know which allele is dominant/recessive to predict phenotypes.
Examples of Genetic Cross Prediction
Crossing two pea plants, one tall (TT) and one short (tt) where T is completely dominant to t.
Proportion of offspring that are short:
a. 0 (none)
b. ¼
c. 1/3
d. ½
e. 1 (all)
Genetic Ratios
Phenotypic Ratios for Simple Genetic Crosses
With complete dominance:
Genotypes of Parents
Phenotypic Ratio of Progeny
Genotypes of Progeny
Aa × Aa
3:1
3/4 A_ : 1/4 aa
Aa × aa
1:1
1/2 Aa : 1/2 aa
AA × AA
Uniform - All showing A
All AA
aa × aa
Uniform - All showing a
All aa
AA × aa
Uniform - All showing A
All Aa
AA × Aa
All AA
All Aa
Genotypic Ratios for Simple Genetic Crosses
Genotypes of Parents
Genotypic Ratio of Progeny
Genotypes of Progeny
Aa × Aa
1:2:1
1/4 AA : 1/2 Aa : 1/4 aa
Aa × aa
1:1
1/2 Aa : 1/2 aa
AA × AA
Uniform - All AA
All AA
aa × aa
Uniform - All aa
All aa
AA × aa
Uniform - All Aa
All Aa
Probability Principles
Definitions
Probability: "the likelihood of the occurrence of a particular event" (Pierce, 2021).
Example: Likelihood of tossing a coin and getting ‘heads’ three times.
To predict outcomes using probability: Need large data sets or knowledge of frequency of events occurring.
For genetic crosses, alleles separate into gametes with equal probability (principle of segregation).
Independent and Mutually Exclusive Events
Independent Events: If the outcome of the first event does not influence the next.
Example: What is the chance of rolling a four the first time and the second time?
Mutually Exclusive Events: If one event excludes the occurrence of another.
Example: What is the chance of rolling either a three or a four?
Conditional Probability
Incorporate additional information into probabilities.
Example: Probability that a tall F2 plant is genotype Tt given that it is x tt.
Understanding subsets of progeny or known phenotypes aids in identifying genotypes.
Test Crosses
Test cross: A cross between an individual with an unknown genotype with a homozygous recessive individual.
Definition - Backcross: A cross between an F1 individual and either of the parental genotypes.
Mendelian Traits
Definition: Traits influenced by a single gene with two alleles (one dominant, one recessive).
Traits studied by Mendel were discrete (two distinct forms) and controlled by a single gene not greatly influenced by environment.
Examples of Mendelian Characteristics and Diseases
Albinism: Caused by OCA2 allele with a large deletion resulting in no melanin production (recessive phenotype).
Location: Chromosome 15q12-q13.1.
Blond Hair: Individuals with two copies of the tyrosinase-related protein 1 (TYRP1) allele with a C to T transition exhibit blond hair (recessive phenotype).
Location: Chromosome 9 p23.
Huntington’s Disease: Caused by CAG nucleotide repeat mutation in the Huntington gene on Chromosome 4; occurs if at least one mutated allele is present (dominant disease).
Cystic Fibrosis: Often caused by a 3 bp deletion in CFTR gene on chromosome 7q31.2; occurs when both alleles carry the mutation (recessive disease).
Interactions Among Genes and Environment
Most traits do not follow simple Mendelian inheritance; environmental factors and complex interactions among genes play a role.
Examples include human traits such as height, weight, skin color, and disease susceptibility governed by multiple alleles and gene interactions.