Genetic Basis of Variation

Explain the terms locus, allele, dominant, recessive, codominant, homozygous, heterozygous, phenotype and genotype.
Explain how genotype is linked to phenotype and how genes are inherited from one generation to the next via the germ cells or gametes.
Explain, with examples, how the environment may affect phenotype.
Use genetic diagrams to solve problems in dihybrid crosses, including those involving sex linkage, codominance and multiple alleles (involving autosomal linkage or epistasis).
Use genetic diagrams to solve problems involving test crosses.
Explain the meaning of the terms linkage and crossing-over and explain the effect of linkage and crossing over on the phenotypic ratios from dihybrid crosses.


Explain, with examples, what is meant by the terms gene mutation and chromosome aberration.
Describe the differences between continuous and discontinuous variation and explain the genetic basis of continuous (many, additive, genes control a characteristic) and discontinuous variation (one or few genes control a characteristic).
Describe the causes of genetic variation in a population.
Describe the interaction between loci (epistasis) and predict phenotypic ratios in problems involving epistasis.
Use the chi-squared test to test the significance of differences between observed and expected results. (The formula for the chi-squared test will be provided.)

Lecture Outline
Introduction
Monohybrid and dihybrid crosses
Codominance
Multiple alleles
Epistasis
Linkage and Crossing-over
Sex determination and sex linkage
Pedigree Study
Chi-squared Test
Polygenic Inheritance
Variation
Mutation

Concept Map
Introduction
(Section 1)
1. INTRODUCTION
What is inheritance?
http://learn.genetics.utah.edu/units/basics/tour/inheritance.swf

Want to know more? Try this…
http://www.dnaftb.org/dnaftb/

Historical development of the gene concept

Mendelian Genetics
Mendel’s Laws
Laws of Inheritance/Heredity
Law of Segregation
Law of Independent Assortment
Law of Dominance

Pea Plant (Pisum sativum)
Significance of pea plants (Pisum sativum) used:
No. of contrasting traits (e.g. tall vs dwarf)
Self and cross pollinating possible
Large no. of seeds
Pea Characteristics
Pea Characteristics
Definition of terms









Monohybrid and Dihybrid crosses
(Section 2)
MONOHYBRID CROSSES
Refers to the inheritance of a single trait
Mendel’s Work :Crosses were made with parents of pure line
Results:
All the F1 resemble the tall parent.
F2 ratio of 3 tall: 1 dwarf.



Mendel’s conclusions:
Some factor present in the pea plants which determine inheritance of height. These factors occurred in pairs.
One factor is dominant and may mask the expression of the other; one factor is recessive and is obscured by the dominant factor.
When gametes are formed, factors separate from each other.

Genetic Basis for Mendel’s Expts


Another eg: Round & Wrinkled-seeded Plants

Test Cross
Simple breeding test, to verify the genotype of an individual. E.g. to determine if an individual is heterozygous.
Applied when a homozygous dominant genotype has the same phenotype as the heterozygous genotype. E.g. Tall phenotype for both TT and Tt
Test cross parent is always homozygous recessive for all genes under consideration.


Example 2:
Albinism:
Recessive mutation
Block in one of the chemical processes leading to production of the pigment melanin
Features: white hair, light-coloured skin and pink eyes
Hidden in heterozygotes ie. carriers





(b) ♀ Carrier x albino ♂
How about the offspring for ♀ Carrier x carrier ♂ ?

(c) ♀ Carrier x carrier ♂


Can you interpret the breeding pattern of the leopard using a genetic diagram ?

Breeding pattern of the leopard:



Breeding pattern of the leopard:


DIHYBRID CROSSES
Inheritance of 2 traits, each specified by a different pair of independently assorting autosomal genes.
Mendel’s work: Crossed plants that differed in 2 pairs of alleles (seed shape and colour).


Results:
All individuals of the F1 generation produced round seeds with yellow cotyledons.

The F2 generation showed the following phenotypes:
9 produce round seeds with yellow cotyledons
3 produce round seeds with green cotyledons
3 produce wrinkled seeds with yellow cotyledons
1 produces wrinkled seeds with green cotyledons

Conclusions:
Based on results Mendel was able to state that the two pairs of characteristics separate and behave independently from one another in subsequent generations. = Law of Segregation

Forms basis of Mendel’s Law on Independent Assortment, which states that any one pair of a characteristic may combine with one of another pair.

Chromosome theory of Heredity
Each pair of factors is carried by a pair of homologous chromosomes, with each chromosome carrying one of the factors.
Since the number of characteristics vastly outnumbers the chromosomes, as revealed by microscopy, each chromosome must carry many factors.
Mendel’s Laws of Heredity
The Law of Segregation:
Each inherited trait is defined by a gene pair.
Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair.
Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization.

The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.


The Law of Dominance:
An organism with alternate forms of a gene will express the form that is dominant.

Mendel’s Laws of Heredity
The Law of Segregation:
Each inherited trait is defined by a gene pair.
Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair.
Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization.
Law of segregation of factors
Related to the separation (segregation) of homologous chromosomes which occurs during Anaphase I of meiosis.


The Law of Independent Assortment:

Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.
Can be explained by the random distribution of alleles into gamete cells.
As a result of independent arrangement and separation of homologous chromosomes, the genotype AaBb gives rise to all four possible combinations of alleles (AB, ab, Ab and aB), which occur equally frequently in the gametes.



Recalling…
Mendel’s work: Crossed plants that differed in 2 pairs of alleles (seed shape and colour).






The 9:3:3:1 (dihybrid) ratio
Depends on 2 conditions:
The 2 different genes must not act on the same character. For instance, if the proteins encoded by the two genes are involved in the same biochemical pathway then the ratios of phenotypes resulting from the genotypes in the F2 generation will be altered.
If 2 genes lie close together on the same chromosome the four classes of gamete are not produced at equal frequencies.
Dihybrid testcross
A test cross can be carried out on the F1 generation to find out which plant has the genotype of RrYy.
RRYY RrYY
RRYy RrYy

Again, is involved crossing the F1 generation with a homozygous recessive pea plant with wrinkled seed and green cotyledon (rryy).


Checkpoint