Aa x aa 9 9 Gametes A , a a 2 A, 2 a all a Mendels first law states that: Factors segregate (i.e separate) from each other during sex cell formation into equal numbers of sex cells. Thinking Vocabulary Sex cells are often referred to as gametes Hereditary factors are now called genes 29 Alternative forms of a gene are called alleles, e.g A is allelic to a. 2 A 2 a 2 A 3 AA 3 Aa 2 a 3 Aa 3 aa all a 2 A 2 Aa 2 a 2 aa
30 A measurable characteristic or distinctive trait is called a phenotype. A phenotype may be visible to the eye, purple, white, yellow, green, may require a form of measurement, e.g height with a ruler, tall and short, weight with a balance or may require special tests e.g human ABO blood groups. A phenotype is due to a gene product or some environmental effect. All genes possessed by an individual constitute that individual's genotype. A genotype is said to be homozygous when two alleles in a diploid are identical e.g AA or aa. A homozygous genotype produces only one type of gamete. Individuals that are homozygous are referred to as pure line or true breeding. A heterozygous genotype is one that carries different alleles e.g Aa. Different kinds of gametes are produced by a heterozygotes. The word hybrid is sometimes used synonymously with a heterozygous condition. A monohybrid is a heterozygote at one gene locus (Aa), a dihybrid is heterozygous at two loci (AaBb), a trihybrid at three loci (AaBbCc) and so on. Whenever one of a pair of alleles can come to phenotypic expression only in homozygous genotype we call that allele recessive. An allele that expresses itself in the heterozygote as well as in the homozygote condition is a dominant allele. Usually a capital letter is used to symbolise a dominant allele and a small letter is used to designate a recessive allele. For example albinism in human beings can be shown as follows: aa an albino homozygous recessive Aa a normal heterozygous person AA is a normal person with homozygous with dominant alleles In cases like albinism we sometimes use the word "carrier" to describe individuals heterozygous for both alleles A and a. We sometimes use the word wild-type to mean the most common allele in a wild population. Usually mutants with a few exceptions are recessive and wild-types are dominant. A + sign is used to symbolise wild-type. The specific location of a gene is called a locus (plural loci) Let us now discuss probability to enable us work out some results expected from Mendelian crosses. 30
31 PROBABILITY will not happen. Outcome of biological events are determined by chance to a very great extent. Consider meiotic production of gametes, the random union of these gametes in fertilisation and so on. Chance determines which of the four meiotic products will become functional, and also determines which one of the millions of sperm cells will fertilise the one mature egg cell in a mammal to take one example.
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- Winter '15
- cells, sperm cells