1. Unit C: Cell Division, Genetics and Molecular Biology
Chapter 19
Section 19.1
Chromosomes and Genetics

2. Chromosomal Theory
It was not until after Mendel, that scientists began to have a true understanding of genetics and heredity.
One of the major discoveries was the chromosome.
It was eventually determined that all organisms had these chromosomes and each chromosome carried some type of genetic information.
Humans have 46 chromosomes, 44 autosomes (non sex determining) and 2 sex chromosomes.

3. Genes on the same chromosome are said to be linked genes.
It was determined that each chromosomes carries thousands of different genes.
Genes on the same chromosome are said to be linked genes.
Over the years the following chromosomal theory of inheritance was devised:
Chromosomes carry genes, the units of heredity.
Paired chromosomes segregate during meiosis. Each sex cell or gamete has half the number of chromosomes found in the somatic (body) cells. This explains why each gamete has only one allele for each gene.

4. Morgan’s Experiments and Sex linked traits
The American, Thomas Hunt Morgan, was among the first of many geneticists who used the tiny fruit fly, Drosophila melanogaster, to study the principles of inheritance.
There are several reasons why the fruit fly is an ideal subject for study.
1. REPRODUCE RAPIDLY
Female Drosophila can reproduce for the first time when they are only 10 to 15 days old, so it is possible to study many generations in a short period of time.
Since genetics is based on probability, the large number of offspring is ideal.

5. 2. SMALL IN SIZE
Many individuals can be housed in a single culture tube without taking up much space.
3. MALE VS. FEMALES
Males can easily be distinguished from females.
Males are smaller and have a rounded abdomen with a dark-coloured posterior (back) segment.
Females are larger and have a pointed abdomen with a pattern of dark bands.
Morgan’s experiments began when he began to notice white-eyed males amidst many red-eye females.
He first crossed a true breeding red-eyed female with a true breeding white-eyed male. His result = all F1 offspring had red eyes.

6. He continued his experiment by crossing two F1 flys.
The F2 generation produced ¾ red eyes and ¼ white eyes.
This all seems pretty normal according to Mendelian genetics; however, Morgan happened to notice that only males ever developed white eyes.
This got him thinking that maybe the patterns of inheritance can differ between males and females.

7. Many years later it was proven that in males because the X and Y chromosomes are not entirely homologous, a difference in inheritance can result.
Traits that are affected by the sex of the organism are known as sex-linked traits.
Sex-Linked Trait – a trait that is determined by genes that are located on the sex chromosomes.
What this meant for Morgan, was that he was correct. Scientists found that the Y chromosome of Drosophila does not carry an allele for the eye colour gene so it can affect inheritance.

8. He was wrong however, about only males being white-eyed.
Some females can have white-eyes it is just so uncommon that he didn’t see it in all his crosses.

9. Sex linked Crosses XRXR x XrY XR Xr XR Xr XR Y XR Y
To write a genotype for a sex-linked trait, the letters X and Y are used.
For example, lets take a look at Morgan’s original cross of a homozygous red-eyed female with a white-eyed male.
XRXR x XrY XR XR
Female (red-eyed) Xr
XR Xr
XR Xr
Male
(red-eyed) Y
XR Y
XR Y

10. Morgan’s f1 and f2 Generations

11. Number of genes on Sex chromosomes
X chromosomes have between genes depending on the species.
Y chromosomes have less than 100 genes.

12. Human sex-linked genes
COLOUR BLINDNESS
Red green colour blindness is recessive allele found only on the X chromosome.
More males have colour blindness because females have to have two X chromosomes with the recessive allele, while males only require the one recessive allele.
HEMOPHILIA
Extremely slow blood clotting.
NIGHT BLINDNESS

13. Recessive Lethal Disorders
Recessive lethal disorders – a trait that, when both recessive alleles are present, it results in death or severe malformation of the offspring.
Recessive lethal disorders occur more commonly in males, because they require only one of the recessive alleles due to their single X chromosome.