It is possible to have two copies of the same gene sequence, one on each homologous chromosome for example, AA, BB, or OO , or two different sequences, such as AB. Minor variations in traits such as those for blood type, eye color, and height contribute to the natural variation found within a species.
The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosomes; other than a small amount of homology that is necessary to reliably produce gametes, the genes found on the X and Y chromosomes are not the same. Prokaryotes have a single loop chromosome, whereas eukaryotes have multiple, linear chromosomes surrounded by a nuclear membrane.
Human somatic cells have 46 chromosomes consisting of two sets of 22 homologous chromosomes and a pair of nonhomologous sex chromosomes. This is the 2 n, or diploid, state. Human gametes have 23 chromosomes or one complete set of chromosomes. This is the n, or haploid, state. Haploid describes a cell that contains a single set of chromosomes. The term haploid can also refer to the number of chromosomes in egg or sperm cells, which are also called gametes. In humans, gametes are haploid cells that contain 23 chromosomes, each of which a one of a chromosome pair that exists in diplod cells.
The number of chromosomes in a single set is represented as n, which is also called the haploid number. In humans when the haploid sperm and egg cell join in fertilisation the resulting zygote has a total of 46 chromosomes the correct number to develop.
By having gametes which are haploid, when the gametes combine, diploid cells are maintained. Also, the mixing of chromosomes in fertilisation is a source of genetic variation. Fertilisation produces a zygote , which will mature into an embryo. The number of cells increases by mitosis, and as the embryo develops, the cells begin to differentiate or specialise. Sexual reproduction, meiosis and gamete formation Sexual reproduction Two parents are needed in sexual reproduction.
We now know that DNA at the same locus on each of a homologous pair of chromosomes can have different information. For each character, an organism inherits two genes, one from each parent. Mendel didn't even know what you know about meiosis. You know that diploid organisms get one of each chromosome from the parents and that's how we get two alleles for each character.
If the two alleles differ, then one, the d ominant allele , is expressed in the organisms appearance. The recessive allele does not show up. The two alleles for each character segregate during gamete production. So if an individual has a dominant allele and a recessive allele, the gametes may get either one; they will separate. The gametes could have either the dominant or the recessive allele.
This is called Mendel's law of segregation. Some more terminology: By convention, we use an upper case letter to represent the dominant allele and a lower case letter to represent the recessive allele.
An individual with two of the same alleles is called homozygous for that character. If an individual possesses two different alleles we say it is heterozygous for that character. Mendel had purple flowering plants that were "true breeders", that is, when self pollinated they always produced purple flowered plants.
The plants were homozygous for the purple allele. The purple flowering plants in the first generation were heterozygous.
They had white recessive alleles. The phenotype of both purple flowered plants was the same; they're genotypes differed. Phenotype can be determined by observation, it is the appearance of an individual.
The genotype is the underlying genetic makeup of an individual. Not all alleles are completely dominant or recessive. An example from the text is the color of snapdragons. When homozygous red flowers RR are crossed with homozygous white flowers rr the F1 generation is all pink.
The colors appear to have "blended", but the genetic material, the genes, have not blended. This is an example of incomplete dominance. Evidence that the genes haven't blended can be found in the f2 generation 9. Dominant alleles don't subdue the recessive alleles.
Consider Tay-Sachs disease. People with the disease can't metabolize a lipid that accumulates in the brain. These people are homozygous recessives tt. Heterozygotes and homozygous dominant individuals Tt, TT appear normal. So we say T is dominant. What's really going on is that the T allele has the genetic information to produce the enzyme necessary to metabolize the deadly lipids. TT individuals make a lot of the enzyme and Tt individuals make some too. TT and Tt individuals make enough, so we call them normal, although if you view this phenomenon at the biochemical level you'll see it as incomplete dominance.
Something to note:Recessive alleles are not always the least common and they are not always the bad ones. There aren't always just two alleles of a gene in the population.
Sometimes there's only one, other times there may be many. A diploid individual can only have two alleles at once of course, but there may be more out there in the population.
The ABO blood groups are a good example. The alleles code for coating on the blood cells. I A produces one type of coat, I B another. Type AB individuals get both. See section 9. Many characters in the real world don't work as neatly as Mendel's peas. Like people's height or skin color. If you looked at a "population" made up of individuals you would find individuals that exhibited a broad range of traits. There are short people, tall people and people of all heights in between.
Thus, variation in height is said to be continuous, and a character, such as height or milk yield of dairy cows, sprint speed of scorpions, etc. Two things help explain these types of characters which exhibit continuous variation.
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