BGYA01 Lecture 12 October 23, 2007
Last class we started our consideration of transmission genetics – the study of how
genes are passed from one generation to the next.
We looked at the Law for segregation of alleles and independent assortment. We also
looked at how you can use your knowledge of these two laws to create Punnett Squares
that allow you to predict the outcomes of particular mating.
If you have attempted the assigned problems you will see that truly understanding
these two laws and their consequences (combine how to correctly use a Punnett Square
or probability theory) is very powerful.
Last class we saw how genotypes give rise to phenotypes WHEN the alleles examined
have only simple dominance /recessive relationships.
Today we are going to see some things that complicate our predictions.
- for some genes both parents don’t have two versions of each gene
- some alleles are not recessive or dominant.
- Sometimes genes interact
Thus in controlled genetic crosses you don’t always get 3:1 phenotypic ratio in the F2 of
a monohybrid cross, and you don’t always get a 9:3:3:1 phenotypic ratio in the F2 of a
Let’s explore this further.
Last class we look at the results of a monohybrid cross (of dominant/recessive alleles)
in which the F1 were selfed to give rise to the F2.
In the F2 that is produced from a self cross of the F1, we expect the Mendelian genotypic
ratio of 1:2:1 (ss:S s:SS)
This 1:2:1 genotypic ratio was obtained because of Segregation of alleles and random
fertilization. In his crosses all offspring received one version of the gene from each
parent, and each parent had two versions of the gene.
These classical Mendelian results occur if the genes in question are autosomal genes.
In fact, up to now, in the crosses we have looked at we have been considering the
inheritance of autosomal genes.
What are autosomal genes?
Autosomal genes are located on the autosomes.
What are autosomes?
Autosomes are the chromosomes of the organism for which the chromosome pairs are
the same in BOTH sexes.
In humans all of us have 22 pairs of homologous chromosomes (chromosome pairs 1-
22) that we call autosomes. Males and females both have 2 of each autosome Figure
9.15, page 195.
Sex chromosomes are specific chromosomes that are different in males and females,
and are important in determining the sex of the individual. (Figure 9.15, page 195 the X
and Y chromosome)
Not all eukaryotic organisms have sex chromosomes. Some plants, and most higher
animals DO have sex chromosomes
In humans we have sex chromosomes.
Normal women have two X chromosomes.
Normal men have one X chromosome, and one Y chromosome.
The X and Y chromosomes are the sex chromosomes in humans.
SEX DETERMINATION IN HUMANS
The sex chromosomes determine the sex of an individual.
In humans the female body plan is the default plan. All of us start out using the female
XX individuals stay with this plan. Lack of a Y means female.
In humans the Y chromosome determines maleness. The Testis Determining Factor TDF
gene is on the Y chromosome.
This gene turns on very early in development, and causes a testis to develop, the testis
produces the hormone testosterone, then the female body plan will be blocked and the
male body plan will develop.
Now let’s get back to inheritance.
Let’s look briefly at the human X and Y chromosomes.
X-linked genes all along the non-psueodoautosomal region of the X chromosome
The very top portion of the X and Y chromosome is homologous and is called the
pseudo-autosomal region (because genes in this region behave like autosomal genes
since the pseudoautosomal region is the same in males and females have).
The pseudoautosomal region is the portion of both the X and the Y chromosome that is
actually the same.
There is a lot of X chromosome beyond the pseudo-autosomal region, and this large
part of the X chromosome contains many important genes.
The human X chromosome has genes on it that relate to sexual development, but the X
chromosome also has lots of genes that relate to other important processes. For
example in humans an important gene for blood clotting is on the X chromosome. The
gene for color vision is also on the X chromosome.
Outside of the pseudoautosomal region, the human Y chromosome has only a few
genes on it, one gene known as the TDF gene(testis determining factor), is very
important in determining maleness. But the Y lacks most of the genes that are on the X
X-linked genes are genes in the non-pseudoautosomal region of the X chromosome
Y-linked genes are on the non-pseudoautosomal region of the Y chromosome. The
only known Y –linked gene is the TDF gene.
Sex-linked genes is a less precise term that refers to genes on a sex chromosome, either
X or Y; but since the Y chromosome only has one Y specific gene, and the X
chromosome has many X-specific genes, the term sex linked gene often is
used(carelessly) to refer to X-linked genes.
THE IMPACT OF X-LINKAGE ON INHERITANCE
Women, since they have 2 X chromosomes, have two versions of all X-linked genes.
Women can be homozygous or heterozygous for X-linked genes.
Men are neither heterozygous, nor homozygous for most of the genes found on the X
chromosome because they have only one X chromosome. Men are called hemizygous
for all of the X-linked genes.
Hemizygosity is the condition of having only one version of a gene, in an otherwise
Because males are hemizygous for X-linked genes, the pattern of inheritance of X-
linked genes is different from autosomal genes.
The situation is similar in Drosophila – an important model system for genetic studies.
The X chromosome has many important genes on it, and the Y chromosome seems only
to have a gene important to male fertility.
So Drosophila males also are hemizygous.
During meiosis the two X chromosomes pair up, during prophase I, then later during
meiosis, two X chromosomes segregate from each other.
Mammalian females (and female Drosophila) donate one X chromosome to all of her
offspring. Thus all gametes from females have an X chromosome.
Last class we started our consideration of transmission genetics the study of how genes are passed from one generation to the next. We looked at the law for segregation of alleles and independent assortment. We also looked at how you can use your knowledge of these two laws to create punnett squares that allow you to predict the outcomes of particular mating. If you have attempted the assigned problems you will see that truly understanding these two laws and their consequences (combine how to correctly use a punnett square or probability theory) is very powerful. Last class we saw how genotypes give rise to phenotypes when the alleles examined have only simple dominance /recessive relationships. Today we are going to see some things that complicate our predictions. for some genes both parents don"t have two versions of each gene some alleles are not recessive or dominant.