Sex Ratio in Humans: In the early 1900’s Thomas Hunt Morgan began breeding a type of fruit fly called Drosophila melanogaster.

Morgan observed the Drosophila’s four pairs of homologous chromosomes – one pair was different in males and females. –In females the chromosomes were identical. – In males the chromosomes were different. One chromosomes looked like those of the female but the other was shorter and hook-shaped.

Morgan called the large chromosome X CHROMOSOME. He called the short, hooked shaped chromosome the Y CHROMOSOME. Morgan correctly identified these chromosomes as the sex chromosomes.

You may want to jot this in the margin of your notes! Why do scientists like to use fruit flies for genetic studies? They have short life spans, many babies, they have just a few chromosomes that are very large!

The presence of HETEROMORPHIC (different shaped) sex chromosomes in one sex of a species, but not the other, provides a potential mechanism for producing equal proportions of male and female offspring.

The actual proportion of male and female offspring is called the SEX RATIO. It can be assessed in two ways. Determining Birth Rates:

The PRIMARY SEX RATIO reflects the proportion of males and females conceived in a population.

The SECONDARY SEX RATIO reflects the proportion that are born. –The secondary sex ratio in the human population is determined using worldwide census data, it does not equal 1.0. Sex ratio by country for total population. Blue represents more women, red more men than the world average of 1.01 males/female.

Sex Ratios and Race: For example in the CAUCASIAN population in the UNITED STATES, the secondary ratio is 1.06 reflecting 106 males born for each 100 females born. In the black population the ratio is In other countries the excess of male births is even higher. In Korea the secondary sex ratio is 1.15.

WHY? There is no experimental evidence that suggest why there are more male births than females. –There has been a speculation that the human Y chromosome is smaller than the X chromosome. The Y bearing sperm are of less mass and therefore more motile. –These sperm may have a greater chance of reaching the egg.

X-Linked: The traits that are determined by genes on the X chromosome are referred to as X- LINKED Traits. Scientists have identified 161 identified loci on the human X chromosome. One example is the inheritance of the common type of color blindness.

This condition is caused by a recessive allele located on the X chromosome. We indicate X chromosome having the two alternative alleles as X C and X c represents the normal and color-blind alleles, respectively.

Linkage cont. In females there are three different genotypes X C X C, X C X c, and X c X c ) in which only the last is color-blind because the normal allele is dominant in heterozygotes. –Males only have one X and a Y. Because the Y chromosome do not carry the color blindness gene. –The males have two possible combinations: X C Y and X c Y normal and colored blind respectively. There is a test for color blindness.

Other Sex-linked traits Hemophilia is another type of sex linked disease in which the blood does not clot normally. Most individuals with X-linked forms of hemophilia are males who have received the hemophilia allele form their unaffected carrier mothers.

History of Hemophilia: It has been noted that hemophilia has afflicted a number of the descendants of Queen Victoria of England.

Czar Nicholas the II’s wife was named Alexandra and was a granddaughter of Queen Victoria. Their son Alexis had hemophilia and was executed with the rest of his family when he was 14 years of age.

X INACTIVATION Females have two alleles for every gene on the X chromosome, while males have only one. There is an inequality in the expression of the genes on the X chromosome. Early in development of a female one X chromosome in each cell is inactivated.

Which X chromosome is turned off in each cell ( the one inherited by mother or the one inherited by the father) is a matter of chance. – As a result a female mammal expresses the X chromosome genes from her father in some cells and those from her mother in others. –Once the X chromosome is inactivated in one cell all the cells that form when that cell divides have the same inactivated X chromosome.

Because the inactivation occurs early in development the adult female has patches of tissue that are phenotypically different in their expression of sex- linked genes. X inactivation alters phenotype NOT GENOTYPE. It is not permanent, because the inactivation reverses in germ-line cells. Fertilization does not have an inactivated X.

X inactivation can be observed at the cellular level because the turned off X chromosome absorbs a stain much faster than the active X chromosome. The nucleus of a female cell in interphase has one dark-staining X chromosome called a BARR BODY ( named after Canadian researcher, Murray Barr).

Why does a normal male cell have no bar body? Only one X chromosome, and is NOT TURNED OFF.

A striking and familiar example of X inactivation is seen in the coat colors of “calico cats”. A sex linked gene confers black, brown, and orange coat color. A male cat with the recessive orange allele has orange fur.

–Coat color in cats is an X-linked gene, with alleles for black and orange-brown, so X B X B and X B Y cats will have a black coat, while X O X O and X O Y will have an orange- brown coat. Another possible combination for female cats would be X B X O. Both of the color alleles would be expressed, so the cat would end up being partially brown and partially black.

A female who is heterozygous for this gene has patches of orange and either brown or black forming a tortoiseshell pattern that reflects different cells expressing the two different alleles.

The earlier the X inactivation the larger the patches of color, because more cell divisions can occur afterward. If white patches also appear, the cat is a calico. Tortoiseshell and calico cats are almost always female.

A gene can sometimes affect another characteristic. This ability of having multiple effects is called pleiotropy. Genes that control fur pigmentation in cats may have an influence on the cats eyes and brain. Pleiotropy

Epistasis

One gene may interfere with the expression of another gene that is independently inherited. This is called epistasis. In flower color a P is required for it to exhibit purple color. PP and Pp = purple colored flowers. This can only happen if a dominant allele is present for another characteristic. PPCc =purple Ppcc = white. The C characteristic has an effect on the color of the flower.

Polygenic Traits Polygenic traits have 3 sets of alleles for a characteristic. Skin color is polygenic. There are 6 genes responsible for this characteristic. BBBBBB= Very dark pigmentation where as bbbbbb = the opposite very light pigmentation. All the other genotypes are intermediates of these combinations.