Principles of Genetics (BIOL2250)

Department of Biology
Memorial University of Newfoundland


Homework Assignment 1:

For complete understanding of dihybrid inheritance complete the following table Ö

Gg Ww produces the following gametes GW, Gw, gW, gw and that the random association of these gametes will result in the following Ö
 
male/female gametes GW Gw gW gw
GW GG WW GG Ww Gg WW Gg Ww
Gw GG Ww GG ww Gg Ww Gg ww
gW Gg WW Gg Ww gg WW gg Ww
gw Gg Ww Gg ww gg Ww gg ww

Homework Assignment 2:

Take four coins of two different types, such as two cents and two nickles.
Let the tails side of the pennies be recorded as "p" and the heads side as "P".
Let the tails side of the nickles be recorded as "n" and the heads side as "N".
Repeatedly shake all four in a cup and drop onto a surface to score.
 

Genes and Chromosomes

Organisms have two copies of most genes (on pairs of chromosomes).
Gene Pairs on separate chromosomes assort independently at meiosis.
Chromosome behaviour at meiosis lead to the observed patterns of inheritance.
Gene pairs separate into gametes (1/2 of each).
When on different chromosomes, genes sort independently but not necessarily when two genes are on the same chromosome.

The Law of Segregation (Mendel's First Law)
Two members of a gene pair segregate from each other into the gametes such that half the gametes carry one member of the pair and half carry the other.

The Law of Independent Assortment (Mendel's Second Law)
Different gene pairs assort independently in formation of the gametes.

How to generate genetic ratios!

1) Punnett Square
The Punnett Square, named after an early geneticist, is constructed by assigning the haploid gametes produced by one parent along the side of the square and those produced by the other along the side.
By filling in the boxes of the square, the genotypes of the offspring are assembled and the genotypic ratio can be determined.
Assigning phenotypes to genotypes, allows the determination of the phenotypic ratio.

2) Branch diagrams
Branch diagrams is another approach to obtain both genotypic and phenotypic ratios.
By setting out the proportions of genotypes or phenotypes for each allele pair and connecting these to proportions of the other allele pairs, a branch or web of genotypes or phenotypes can be constructed

3) Simple Statistical Rules
By applying the Product Rule and the Sum Rule, the genotypes and phenotypes arising from a cross can be calculated.

The Product Rule says that the probability of independent events occurring together is the product of the probabilities of the individual events.

The Sum Rule says that the probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.

For example: the probability of an individual being homozygous recessive at four loci (aa bb cc dd)
arising from heterozygous parents (Aa Bb Cc Dd X Aa Bb Cc Dd) is

1/4 X 1/4 X 1/4 X 1/4
which is 1/44
or 1/256.
The probability of NOT being homozygous recessive at four loci is (1-1/256) or 255/256 or P= 0.9960!!!

Dominant versus recessive alleles.

Consider the following case of a simple functional gene product (perhaps an enyzme or another protein type),
Such that
The A allele leads to a gene product and or function and
The a allele  results in no gene product and or function.
Quite often, the recessive allele of a gene is this way because the gene's usual function is either gone or reduced.
Thus the dominant allele is dominant because a gene product is produced so that a heterozygote will display the phenotype associated with the gene's active state.

An amorph is a recessive allele of a gene that results in no gene activity.
A hypomorph is a recessive allele of a gene that results in significantly reduced no gene activity.

However, not all dominant alleles of genes are the usual gene state.

A hypermorph is a dominant allele of a gene that results from increased gene activity.
An antimorph is a dominant allele of a gene that results in gene activity that counters the activity of the other allele.

Pedigrees of Mendelian autosomal dominant disorders show affected males and females in each generation;
they also show that affected men and women transmit the condition to equal proportions of their sons and daughters.
 

Sex chromosomes and Sex Linkage

Sexual Dimorphism (having two genders, male and female) is a common property of plants and animals.
Genes that are located within the sex chromosomes have unique inheritance patterns.
Sex-linked inheritance regularly shows different phenotypic ratios in the two sexes of progeny, as well as different ratios in reciprocal crosses.
Inheritance patterns with an unequal representation of phenotypes in males and females can locate the genes concerned to one or both of the sex chromosomes.

Sex chromosomes, in humans and Drosophila melanogaster (the fruit fly), are X and Y.
The rest of the chromosomes are the autosomes.

Humans, with a total of 46 chromosomes, have 22 pairs of autosomes and either a pair of X chromosomes (in females) or and X and a Y (in males).
In humans (and fruit flies) the XX female is the homogametic sex and the XY males are the heterogametic sex.
This definition comes from the fact that females make all the same type of gamete (X-chromosome bearing ones) while males make both X chromosome and Y chromosome bearing gametes.
In some birds, the males are the homogametic sex and females are the heterogametic sex (the WW / WZ system; WW are males, and WZ are females).
Rarely, changes to the above result in alternative complements of sex chromosomes.
This reveals a difference in the way sexes are determined in flies and humans.

The sex chromosomes have homologous and nonhomologous regions.
The differential regions of one sex chromosome have no counterpart regions (or genes) present within the other sex chromosome.
Hemizygous refers to genes in the region present on one chromosome but not the other one of a pair.
These genes display sex linkage.

X chromosome Inactivation
In female mammals (and humans), one X chromosome becomes highly condensed and becomes a Barr body early in development.
The selection of chromosome to become the Barr body is a random event.
As a result, females are a mosaic of tissues that have one of the two X chromosomes inactivated.
When a colour gene is associated with such a process, the effect is obvious.
An X-linked condition in humans, anhidrotic ectodermal dysplasia which results in no sweat glands in a hemizygous male, produces sectors of absent sweat glands.

Y chromosome Inheritance
Few genes are known to reside on the Y chromosome.
However, testis-determining factor (TDF) is present within the human Y chromosome.

Medical Genetics

Pedigree analysis is fundamental to understanding the influence of genetics upon the human condition.
In human pedigrees, an autosomal recessive disorder is revealed by the appearance of the disorder in the male and female progeny of unaffected persons.
Autosomal recessive disorders are very common and include phenylketonuria (PKU), the inability to convert phenylalanine (an essential amino acid present in all dietary proteins) to tyrosine.

Autosomal dominant disorders include pseudo-achondroplasia (a type of dwarfism), Polydactyly (extra digits) and Brachydactyly  (very short fingers).
Also, Huntington Disease is a late onset neural degeneration disease is a autosomal dominant.

X-linked recessive disorders include the common red-green colour-blindness, hemophilia (present in the Royal Family) and Duchenne muscular dystrophy.
With X-linked recessive conditions, many more males than females display the disease and the female offspring of an affected male are not (usually) affected but their sons (one-half) are.

X-linked dominant are very rare in humans (ie. hypophosphatemia) and affected males pass the condition only to their daughters who may pass this on to both sons and daughters.

Populations of plants and animals (including humans) are highly polymorphic.
This means that multiple alleles for many genes are present in a population.
Contrasting morphs are generally determined by alleles inherited in a simple Mendelian manner.

An allelic series for a number of alleles of a single gene may be generated by observing the effects of multiple pair-wise combinations of different versions of the gene.

w+w+   =  w+w  >>  wawa   =  waw >> ww


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