Genetic Diseases and carriers
A genetic disease is an illness that is caused by a gene. There are more than 400 genetic diseases that can occur in humans and most of these are caused by recessive alleles. Therefore, these diseases only develop if the individual is homozygous for the recessive gene. If a person is heterozygous with a dominant gene and the recessive gene carrying the genetic disease, they will not show symptoms of the disease, however they can pass the recessive gene to their offspring. The parent is therefore a carrier.
Genetic diseases caused by a recessive allele usually appear unexpectedly. Both parents must be carriers but since they do not show symptoms they are unaware of this. The probability of these parents having a child with the disease is 25 percent. A small proportion of genetic diseases are caused by a dominant alle. It is not possible to be a carrier of these diseases because if the person has one dominant allele they will develop the symptoms. If one parent has the allele of the disease, the chance of the child inheriting the disease is 50 percent.
Most genetic diseases reduce the chances of survival and reproduction, so the alleles causing them are rare and barely passed to a offspring. There are a small number of genetic diseases where the frequency of the allele that causes them is much higher. In these cases, the alleles that give an advantage, causing the frequency to increase by natural selection.
Thomas Morgan was one of a group of genetics that realized the importance of Mendel’s experiments and investigated the pattern of inheritance of otehr species to find out of it works in other species. Morgan chose to study the fruit fly Drosphila. Its life cycle is 2 weeks which allows crossing experiments to be done quickly with many flies. Many of Morgan’s experiments showed the same pattern of inheritance as pea plants. Firstly, two alles of each gene are present. These two alleles can either be homozygous or heterozygous. Just one allele is passed on to offspring in gametes. Lastly, one allele is usually dominant over another allele.
Unlike pea plants Drosphila, has separate males and females and this allowed Morgan to preform reciprocal crosses. Examples of these included crossing normal winged males with small winged females and small winged males with normal-winged females.
In most cases, the ratios were the same whichever way the cross was done but in some cases it was different. The first example of this in Drosphila involved in eye color. This pattern of inheritance, where there are differences in genotypes and phenotypic rations between males and females is called sex linkage. Geneticists had observed that the inheritance of genes and of chromosomes showed clear parallels which indicated that the genes were probably located on chromosomes. The female Drosphila had two X chromosomes while males only had one copy. Morgan concluded that sex linkage of eye color could therefore be caused by the eye color gene being located on the X chromosome. Male Drosphila also have a Y chromosome, but this gene does not carry the eye color gene. This similar pattern of sex linkage due to a gene on the X chromosome has been discovered in humans. Hemophilia, or red-green color blindness is a very common sex linkage condition.
Determination of gender in humans
The gender of an individual is determined at the time of fertilization by one chromosome carried in the sperm. This can either be a Y or X chromosome, and because they determine the sex of an individual, they are called sex chromosomes. The X chromosome is large with many genes important to the development in both males and females. The Y chromosome is much smaller and has fewer genes. Part of the Y chromosome has the same sequence of genes as a small part of the X chromosome, but some of the genes on the rest of the Y chromosome is not on the X chromosome because it is not needed to female development. An example of this is TDF which is only found on Y chromosomes. It starts the development of male features such as testes and testosterone production. Therefore, males have one Y chromosome for male development and one X chromosome for other vital genes. Females have two X chromosomes and because they do not have the TDF gene on a Y chromosome, they develop ovaries instead of testes and female sex hormones are produced. Females pass one X chromosome in each egg cell so all offsprings inherit a X chromosome. However, when sperms are formed, half contain X chromosomes while the other half contain Y chromosomes so the chances of inheriting both are half. If a Y chromosome is inherited, a son is produced but if a X chromosome is inherited, a daughter is produced.
Haploid and Diploid
In the body cells of most eukaryotes, there are two chromosomes of each type, which are called two homologous chromosomes. This is a diploid cell. Experiments in genetics show that parents only pass on one copy of each gene to their offspring because only one gene is passed on in a gamete. Since their is only one chromosome of each type in a gamete, it is haploid. When a male gamete and female gamete fuse in fertilization, the zygote that is produced is diploid. Division of the zygotes results in the production of more cells due to mitosis. All the cells in our body are therefore identical and haploid. To produced haploid gametes, a special type of division is needed, called meiosis.
Data-based questions: hemophilia in Queen Victoria’s family
1. There are no male carriers as this condition is sex linked and a recessive trait. Only if they have two recessive genes would they be affected. This condition occurs on the X chromosome, and because males have one X and Y, if they had the gene on their X chromosome, they would be affected automatically. They can not be carriers as they either have it on their X chromosome and are affected, or they do not have it on their chromosome and are not affected.
2. Because this is a recessive trait, if there is a dominant allele present, hemophilia would not affect the person. Only when there are two recessive aleles of the gene would the female be affected. This would require passing the gene from a carrier mother and a affected father which is very rare. Therefore there are no females on this chart.
3. It is sex-linked because only males are affected. This suggests that the trait came from the female carrier as a normal male is not affected. Because males only need one X chromosome as they also have a Y chromosome, if they inherit the gene from their mother, the condition will be expressed. Since males are affected even if the father has not been, this trait is sex linked.
The chances of a daughter being a carrier is 50% and of the daughter not affected is 50%.
5. Victoria Eugenia
There is a 50% chance that the second son is affected and 50% chance that the third son is affected. However, the normal male symbols indicate that neither are affected therefore their alleles are XCBY.