Gg Ww produces the following gametes GW, Gw, gW, gw and that the random
association of these gametes will result in the following Ö
|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|
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.
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
The probability of NOT being homozygous recessive at four loci is (1-1/256) or 255/256 or P= 0.9960!!!
An amorph is a recessive allele of a gene that results in no
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, 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.
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.
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