Science at a Distance

Mendelian Genetics

Lecture Notes

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Mendel - Part Three

Independence of Traits

Mendel observed that one trait (such as flower color) was independent of a second trait (such as seed color).

Mendel showed great insight when he concentrated on studying one trait at a time.
He saw that a plant could have red flowers and green or yellow seeds.
Flower color was independent of seed color.
One trait did not seem to influence the expression of the second trait.

Independent Inheritance of Traits - Starting Material

Mendel performed a series of experiments to show that the inheritance pattern of one trait (such as flower color) was independent of the inheritance pattern of a second trait (such as seed color, or plant height).

Mendel created parental plants that were pure breeding for two, independent traits.
For example, he studied the traits for flower color (red or white) and the height to which plants would grow (tall or short).
One plant was homozygous for both the normal genes for these traits (for example RR and TT). This plant was red flowered and tall.
The other plant was homozygous for both mutant forms of these traits (rr and tt). This plant was white flowered and short.
These two plants became the starting (parental) generation for a two-factor cross.

Two Factor Crosses - Round One

In the first round of crosses between plants differing in two traits Mendel took the pollen from one parent and used it to fertilize the egg cells of the second plant.

Both plants produce haploid sex gametes by meiosis.
Every gamete must receive one of each gene (for flower color and for height).
All the sex gametes must, therefore have a single flower color gene and a single height gene.
The plant with the RR:TT genotype produces haploid sex gametes that have the genes R:T.
The plant with the rr:tt genotype produces haploid sex gametes that have the genes r:t.
At fertilization, two of these sex gametes fuse.
A R:T gamete fuses with a r:t gamete producing a diploid zygote with the combination of genes Rr:Tt.
When the seeds containing these zygotes grow into the new F1 plants the following year, they all have the phenotype Red Flowers and Tall height.

Two Factor Crosses - Round Two

During the second round of crosses, Mendel took the F1 plants with the heterozygous genotypes generated in round one, and crossed them again to produce the F2 generation.

Mendel took as parents for the second round of crosses two of the F1 plants.
These two parents were genetically identical (Rr:Tt).
Each plant produced haploid sex gametes by meiosis.
Each sex gamete must have one flower color gene and one height gene.
There are four possible combinations of flower color and height genes that can occur in each sex gamete; R:T, R:t,r:T, or r:t.
The separation of chromosomes carrying these genes into sex gametes is random.
The genes (and chromosomes) assort themselves independently of each other. This is called Independent Assortment.
Any sex gamete from one plant can fertilize any sex gamete from the other parent.
A visual representation of the possible gene combinations in the F2 zygotes is called a Punnett Square.

Growing the F2 Generation

Mendel grew the seeds that would give him the F2 generation. When the plants were fully mature he examined all the phenotypes, and found some new ones.

Among the F2 generation plants, Mendel found some that had red flowers and were tall, and some that had white flowers and were short. These were exactly the phenotypes of the original parents.
Then he found some F2 plants that had red flowers and were short in height.
He also found some F2 plants that had white flowers and were tall in height.
These two categories of phenotype were new.
After counting all the plants, he worked out the ratios; there were 9 plants that were red/tall, 3 plants that were red/short, 3 plants that were white/tall and 1 plant that was white/short.
From these results, Mendel was sure that the inheritance of the genes for flower color and height were independent of each other.

Sex Determination

Long after Mendel and his work, other scientists began work on the genetic determination of other traits. One of these traits was sex gamete production, how it is controlled and how it is determined.

Gamete production is an important part of the sexual life cycle of all diploid creatures.
In multicellular organisms (like ourselves) meiosis takes place in specialized organs of the body where haploid cells are produced and mature into sex gametes.
Female sex gametes are large, produced in small numbers and cannot move on their own.
Male sex gametes are small, produced in large numbers and can move with the help of long flagella.
In many creatures, the individual that produces only male sex gametes is called the male, whereas the individual that produces only female sex gametes is the female.
At the time of fertilization a male gamete fuses with a female gamete to produce a zygote that grows into the next generation.
Sexual differentiation into "Male" and "Female" is an important phenotypic trait that is genetically regulated.

The X and Y Chromosomes

In humans and fruit flies, the genetic determination of sexual identity involves whole chromosomes.

Humans have 46 chromosomes.
44 of these chromosomes pair together to make 22 pairs of homologous chromosomes (called autosomes) that carry most of the genes that humans need for the determination of their phenotype.
Two chromosomes, called the sex chromosomes, are different, because they are involved in determining the sex of the individual carrying them.
An individual who is phenotypically female carries two copies of the X chromosome (XX).
An individual who is phenotypically male carries one X chromosome and one Y chromosome (XY).
It is now known that it is the presence of the Y chromosome that makes the individual male.

Sex Inheritance

During meiosis, the sex chromosomes (X and Y) pair and are separated into different gametes. Fusion of two gametes at the time of fertilization produces a zygote, who's sex is determined by the combination of chromosomes inherited.

Females with the XX pair of chromosomes can only produce gametes that contain X chromosomes.
Males with the XY pair of chromosomes can produce two different kinds of gametes in equal numbers; those with an X chromosome and those with a Y chromosome.
A Punnett square shows the possible combinations of chromosomes in the next generation.
In each generation there are equal numbers of males and females produced by this system.

Sex-linked Traits

In humans, the X chromosome carries some genes that are not found on the Y chromosome. Inheritance of the phenotypic traits determined by these genes is therefore linked to the sex of the individual.

The Y chromosome is important in the determination of the sex of the individual.
The X chromosome carries other genes which are not associated with the determination of sex. One of these genes codes for a protein needed for the correct clotting of blood after a wound.
The Y chromosome in humans does NOT carry this gene.
Mutations can occur in this blood clotting gene, resulting in a blood protein that cannot clot the blood properly after a wound.
Improper blood clotting is a medical condition known as hemophilia.
There are three types of possible chromosomal states; XH (X chromosome with a normal blood clotting gene), Xh (X chromosome with a mutant blood clotting gene) and Y0 (Y chromosome with no blood clotting gene at all).
There are five possible types of individual carrying these chromosomes and genes; XHXH (female/normal clotting), XHXh (female/normal clotting/carrier of the trait), XhXh (female/hemophiliac/rare condition until very recently), XHY0 (male/normal), and XhY0 (male/hemophiliac).

The Inheritance of Sex-linked Traits

The pattern of inheritance of genes and traits carried by the X chromosome closely follows the inheritance of the sex of the individuals.

Hemophilia is a medical condition which, until recently, was incurable and usually fatal.
Until recently, it was a condition found almost exclusively in men.
Today medical science can treat hemophiliacs by replacing the missing blood proteins, but it cannot cure them.
APunnett square shows the inheritance pattern of a male individual having children with a female individual who carries one X chromosome with a damaged (mutant) blood clotting gene.
A Punnett square shows the inheritance pattern of a male hemophiliac having children with a female who has no damaged genes.

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Lecture Notes - Part 1	Lecture Notes - Part 2
Mendelian Genetics - Topic Outline