Genes that are on the Y chromosome are said to be Y-linked. Parents pass on traits or characteristics, such as eye colour and blood type, to their children through their genes. Some health conditions and diseases can be passed on genetically too. Sometimes, one characteristic has many different forms.
Changes or variations in the gene for that characteristic cause these different forms. These two copies of the gene contained in your chromosomes influence the way your cells work. The two alleles in a gene pair are inherited, one from each parent. Alleles interact with each other in different ways. These are called inheritance patterns. Examples of inheritance patterns include:. An allele of a gene is said to be dominant when it effectively overrules the other recessive allele.
The allele for brown eyes B is dominant over the allele for blue eyes b. So, if you have one allele for brown eyes and one allele for blue eyes Bb , your eyes will be brown. This is also the case if you have two alleles for brown eyes, BB. However, if both alleles are for the recessive trait in this case, blue eyes, bb you will inherit blue eyes. For blood groups, the alleles are A, B and O.
The A allele is dominant over the O allele. Blood group A is said to have a dominant inheritance pattern over blood group O. If the father has two O alleles OO , he has the blood group O. For each child that couple has, each parent will pass on one or the other of those two alleles. This is shown in figure 1. This means that each one of their children has a 50 per cent chance of having blood group A AO and a 50 per cent chance of having blood group O OO , depending on which alleles they inherit.
The combination of alleles that you have is called your genotype e. The observable trait that you have — in this case blood group A — is your phenotype. If a person has one changed q and one unchanged Q copy of a gene, and they do not have the condition associated with that gene change, they are said to be a carrier of that condition.
The condition is said to have a recessive inheritance pattern — it is not expressed if there is a functioning copy of the gene present. If two people are carriers Qq of the same recessive genetic condition, there is a 25 per cent or one in four chance that they may both pass the changed copy of the gene on to their child qq, see figure 2.
As the child then does not have an unchanged, fully functioning copy of the gene, they will develop the condition. There is also a 25 per cent chance that each child of the same parents may be unaffected, and a 50 per cent chance that they may be carriers of the condition. Half of these chromosomes come from one parent and half come from the other parent.
Under the microscope, we can see that chromosomes come in different lengths and striping patterns. When they are lined up by size and similar striping pattern, the first twenty two of the pairs these are called autosomes; the final pair of chromosomes are called sex chromosomes, X and Y. The sex chromosomes determine whether you're a boy or a girl: females have two X chromosomes while males have one X and one Y.
But not every living thing has 46 chromosomes inside of its cells. For instance, a fruit fly cell only has four chromosomes! Each gene has a special job to do. The DNA in a gene spells out specific instructions—much like in a cookbook recipe — for making proteins say: PRO-teens in the cell. Proteins are the building blocks for everything in your body. Bones and teeth, hair and earlobes, muscles and blood, are all made up of proteins. Those proteins help our bodies grow, work properly, and stay healthy.
Scientists today estimate that each gene in the body may make as many as 10 different proteins. That's more than , proteins! Like chromosomes, genes also come in pairs. Each of your parents has two copies of each of their genes, and each parent passes along just one copy to make up the genes you have. Genes that are passed on to you determine many of your traits, such as your hair color and skin color. Maybe Emma's mother has one gene for brown hair and one for red hair, and she passed the red hair gene on to Emma.
If her father has two genes for red hair, that could explain her red hair. For instance, in mice, experiments involving pigment cells have shown that pigmentation plays a role in maintaining fluid in ear canals. Animals that lack the pigment also lack ear canal fluid, which causes their ear canals to collapse. In turn, this collapse contributes to degeneration of the auditory nerves, which results in deafness Sunquist, Not all instances of pleiotropy are so straightforward, however.
For example, in humans, the p53 gene directs damaged cells to stop reproducing, thereby resulting in cell death. This gene helps avert cancer by preventing cells with DNA damage from dividing, but it also suppresses the division of stem cells , which allow the body to renew and replace deteriorating tissues during aging Rodier et al.
This situation is therefore an example of antagonistic pleiotropy, in which the expression of a single gene causes competing effects, some of which are beneficial and some of which are detrimental to the fitness of an organism.
The idea of antagonistic pleiotropy is central to the theory of aging proposed by American biologist G. Williams in In particular, Williams suggested that while some genes, like p53 , increase the odds of successful reproduction and fitness early in life, they actually decrease fitness later in life. Moreover, because the gene's harmful effects appear after reproduction is complete, the gene is not eliminated through natural selection.
Yet another example of antagonistic pleiotropy can be found in Drosophila. It is still not known exactly which genes determine this fecundity-mortality connection. Nevertheless, this example highlights the idea that antagonistic pleiotropy can be a trade-off between beneficial and detrimental effects.
As touched upon earlier in this article, there are many examples of pleiotropic genes in humans, some of which are associated with disease. For instance, Marfan syndrome is a disorder in humans in which one gene is responsible for a constellation of symptoms, including thinness, joint hypermobility, limb elongation, lens dislocation, and increased susceptibility to heart disease.
Similarly, mutations in the gene that codes for transcription factor TBX5 cause the cardiac and limb defects of Holt-Oram syndrome , while mutation of the gene that codes for DNA damage repair protein NBS1 leads to microcephaly, immunodeficiency, and cancer predisposition in Nijmegen breakage syndrome. One of the most widely cited examples of pleiotropy in humans is phenylketonuria PKU. This disorder is caused by a deficiency of the enzyme phenylalanine hydroxylase, which is necessary to convert the essential amino acid phenylalanine to tyrosine.
A defect in the single gene that codes for this enzyme therefore results in the multiple phenotypes associated with PKU, including mental retardation, eczema, and pigment defects that make affected individuals lighter skinned Paul, The phenotypic effects that single genes may impose in multiple systems often give us insight into the biological function of specific genes.
Pleiotropic genes can also provide us valuable information regarding the evolution of different genes and gene families, as genes are "co-opted" for new purposes beyond what is believed to be their original function Hodgkin, Quite simply, pleiotropy reflects the fact that most proteins have multiple roles in distinct cell types; thus, any genetic change that alters gene expression or function can potentially have wide-ranging effects in a variety of tissues.
Chang, B. Retinal degeneration mutants in the mouse. Vision Research 42 , — Fairbanks, D. Mendelian controversies: A botanical and historical review. American Journal of Botany 88 , — An analysis of the pleiotropic effects of a new lethal mutation in the rat Mus norvegicus. Hartl, D. Genetics: Analysis of Genes and Genomes , 6th ed. Boston, Jones and Bartlett, Hodgkin, J. Seven types of pleiotropy. International Journal of Developmental Biology 42 , — Landauer, W.
Weight and size of organs in frizzle fowl. Storrs Agricultural Experiment Station Bulletin Paul, D. Skin color is another trait that is very obvious in humans that is controlled by many, many different genes. And this is why you get differences between parents and children in skin color, although they tend to resemble one another. Polygenic traits are quite different from the classical Mendelian trait in where we see that one gene controls one characteristic or one phenotype. Surprisingly, most traits in humans, and in fact most traits in most organisms, are polygenic.
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