Only girls receive X chromosomes from their fathers. Queen Victoria of England was a carrier of the gene for hemophilia. She passed the harmful allele for this X-linked trait on to one of her four sons and at least two of her five daughters. Her son Leopold had the disease and died at age 30, while her daughters were only carriers. As a result of marrying into other European royal families, the princesses Alice and Beatrice spread hemophilia to Russia, Germany, and Spain.
By the early 20th century, ten of Victoria's descendents had hemophilia. All of them were men, as expected. Queen Victoria with her husband and nine children in By comparison to the X chromosome, the much smaller Y chromosome has only about 26 genes and gene families. Most of the Y chromosome genes are involved with essential cell house-keeping activities 16 genes and sperm production 9 gene families.
Only one of the Y chromosome genes, the SRY gene, is responsible for male anatomical traits. When any of the 9 genes involved in sperm production are missing or defective the result is usually very low sperm counts and subsequent infertility. One in six American couples are infertile. Because the Y chromosome only experiences recombination with the X chromosome at the ends as a result of crossing-over , the Y chromosome essentially is reproduced via cloning from one generation to the next.
This prevents mutant Y chromosome genes from being eliminated from male genetic lines except by inactivation or deletion. Subsequently, the Y chromosome now has few active genes and mostly contains genetic junk rather than genes. Chimpanzees are our closest living relatives. They have been on a separate evolutionary path from humans for only million years. The genes are like a series of letters strung along each edge. These letters are used like an instruction book.
The letter sequence of each gene contains information on building specific molecules such as proteins or hormones — both essential to the growth and maintenance of the human body. Although every cell has two copies of each gene, each cell needs only certain genes to be switched on in order to perform its particular functions.
The unnecessary genes are switched off. A change in a gene can occur spontaneously no known cause or it can be inherited. Changes in the coding that makes a gene function can lead to a wide range of conditions. Humans typically have 46 chromosomes in each cell of their body, made up of 22 paired chromosomes and two sex chromosomes.
These chromosomes contain between 20, and 25, genes. New genes are being identified all the time. The paired chromosomes are numbered from 1 to 22 according to size. Chromosome number 1 is the biggest. These non-sex chromosomes are called autosomes. People usually have two copies of each chromosome. One copy is inherited from their mother via the egg and the other from their father via the sperm.
A sperm and an egg each contain one set of 23 chromosomes. When the sperm fertilises the egg, two copies of each chromosome are present and therefore two copies of each gene , and so an embryo forms. The chromosomes that determine the sex of the baby X and Y chromosomes are called sex chromosomes. A person with an XX pairing of sex chromosomes is biologically female, while a person with an XY pairing is biologically male. As well as determining sex, the sex chromosomes carry genes that control other body functions.
There are many genes located on the X chromosome, but only a few on the Y chromosome. Genes that are on the X chromosome are said to be X-linked. 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.
Uppercase B, uppercase E is labeled to the left of the top quadrant; lowercase b, lowercase e is labeled outside the second left quadrant; uppercase B, lowercase e is labeled outside the third left quadrant; and lowercase b, uppercase E is labeled outside the fourth left quadrant.
On the upper right, the male parent genotype is also uppercase B lowercase b, uppercase E lowercase e. Uppercase B, uppercase E is labeled to the right of the top quadrant; lowercase b, lowercase e is labeled to the outside the second right quadrant; uppercase B, lowercase e is labeled outside the third right quadrant, and lowercase b, uppercase E is labeled outside the fourth right quadrant.
The offsprings' genotype and phenotype is represented in each of the cells of the Punnett square. Nine of the 16 cells contain brown-bodied flies with red eyes.
Of these nine flies, one has the genotype uppercase B, uppercase B, uppercase E uppercase E; four have the genotype uppercase B lowercase b, uppercase E lowercase e; two have the genotype uppercase B uppercase B, uppercase E lowercase e; and two have the genotype uppercase B lowercase b, uppercase E uppercase E.
Three cells contain brown-bodied flies with brown eyes. Of these three flies, one has the genotype uppercase B uppercase B, lowercase e lowercase e and two have the genotype uppercase B lowercase b, lowercase e lowercase e. Three cells contain black-bodied flies with red eyes. Of these three flies, one has the genotype lowercase b lowercase b, uppercase E uppercase E and two have the genotype lowercase b lowercase b, uppercase E lowercase e.
The final cell contains a black-bodied fly with brown eyes; this fly has the genotype lowercase b lowercase b, lowercase e, lowercase e.
In another example of Mendel's independent assortment principle, a test cross between a heterozygous BbEe fly and a homozygous bbee fly will yield offspring with only four possible genotypes BbEe, Bbee, bbEe, and bbee and four possible phenotypes brown body with red eyes, brown body with brown eyes, black body with red eyes, and black body with brown eyes , as shown in Figure 2.
Thus, in this case, the ratio of phenotypes observed among the offspring will be 1 brown body, red eyes : 1 brown body, brown eyes : 1 black body, red eyes : 1 black body, brown eyes. On the upper right, the male parent genotype is lowercase b lowercase b, lowercase e lowercase e. Lowercase b lowercase e is labeled to the right of the top quadrant, the second right quadrant, the third right quadrant, and the fourth right quadrant.
Four of the 16 cells contain brown-bodied flies with red eyes. All four flies have the genotype uppercase B, lowercase b, uppercase E lowercase e. Four cells contain brown-bodied flies with brown eyes. These flies have the genotype uppercase B lowercase b, lowercase e lowercase e. Four cells contain black-bodied flies with red eyes.
These flies have the genotype lowercase b lowercase b, uppercase E lowercase e. The four remaining cells contain black bodied flies with brown eyes; these flies have the genotype lowercase b lowercase b, lowercase e, lowercase e. Exceptions to independent assortment. However, because the alleles BV and bv are linked, the observed phenotypic ratio is much different than the expected ratio.
What is the lesson to be learned from the body color-wing length example? In short, whenever two genes are linked because of their location on a chromosome, their alleles will not segregate independently during gamete formation.
As a result, test crosses involving alleles of linked genes will yield phenotypic ratios that stray from the classic Mendelian ratios. Also in the case of linked genes, the phenotypic ratio will show higher numbers of offspring with the parental genotypes than offspring with the recombinant genotypes.
Make your own fly. Thomas Hunt Morgan. The fly geneticist and his remarkable findings. Key Questions Who discovered gene linkage? What is sex linkage in flies? How can we use linkage to map genes in a chromosome?
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