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9 Genes, chromosomes and patterns of inheritance. The genetic lottery. Genetic instructions are packaged in gametes (either eggs or sperm cells ) for transmission from parents to offspring.
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The genetic lottery • Genetic instructions are packaged in gametes (either eggs or sperm cells) for transmission from parents to offspring. • Each parent contributes a set of genetic instructions to its offspring so that the offspring inherits a double set. • Genetic instructions consist of genes that are made of DNA. • The genetic instructions comprise the genotype of an organism. • The phenotype is the physical, biochemical or physiological expression of the genotype. • In animals, egg and sperm cells are formed in specialised organs called gonads — ovaries in females and testes in males.
Figure 9.11 Various diagrammatic representations of chromosomes (a) in the double-stranded condition, and (b) in the single-stranded condition. The double-stranded condition is a temporary state formed only during part of a cell multiplication cycle (mitosis) or during gamete formation (meiosis).
Chromosomes are organised collections of genes. • Each species has a characteristic number of chromosomes, known as the diploid number, typically present in body (somatic) cells. • Human chromosomes in body cells exist in pairs, normally 23 pairs. • The 23 pairs of human chromosomes include 22 pairs of autosomes present in both sexes, and one pair of sex chromosomes, XX in the female and XY in the male. • Additional or missing entire chromosomes or parts of chromosomes typically produce deleterious effects. • Various mechanisms of sex determination exist in Kingdom Animalia.
Meiosis: source of variability in offspring • During meiosis, gametes with a haploid set of chromosomes are formed. • Normal human eggs contain 22 different autosomes and one X chromosome. • Normal human sperm contains 22 different autosomes and either one X chromosome or one Y chromosome. • Normally only one member of each pair of chromosomes goes into a gamete. • Variation between gametes of one person arises from exchanges during crossing over and from random disjoining of chromosomes. • Non-disjunction leads to gametes with abnormal numbers of chromosomes.
Genes: inherited instructions • An estimated 20 000 to 25 000 human genes exist. • Genes are identified in terms of the functions they control. • A system of symbols to denote genes exists. • Excluding mitochondrial genes, genes are located on chromosomes in the nucleus of eukaryotic cells. • Each gene occupies a specifi c position or locus on a chromosome. • Genes located on the same chromosome form a linkage group. • A very small number of genes exist in mitochondria.
Alleles: particular forms of a gene • One gene can exist in a number of different forms, called alleles. • Each different allele is identified by its specific phenotypic action. • A person is said to be homozygous when she/he has two identical alleles and heterozygous when she/he has two different alleles of one gene. • Males are hemizygous for both X-linked and Y-linked genes.
When an allele is expressed in the phenotype of a heterozygote organism, the trait controlled by that allele is dominant. • The trait controlled by the ‘hidden’ allele in a heterozygote is recessive. • When two different alleles are both expressed in the phenotype of a heterozygous organism, they are said to show co-dominance. • The phenotype of an organism can be due to an interaction between the genotype and the environment.
Rules of the genetic game-dihybrid cross Figure 9.45 Punnett square showing the cross of Tracey and John. Note that, in the overall summary statement below the Punnett square, the ‘dash’ symbol (–) means that the second allele in the genotype can be either of the two possible alleles. So, for example, A– denotes both AA and Aa.
Test crosses • Another important type of cross that can be either monohybrid or dihybrid is a test cross. A test cross involves one parent that is homozygous recessive, such as aa or aabb, and is used for the following purposes: • to identify if an organism showing a dominant trait is homozygous or heterozygous • to establish linkage relationships between genes. • In reality, today, it is possible to make these decisions using molecular techniques.
Because of equal numbers of each kind of offspring, we can conclude that the genesare not linked but are assorting independently
The chance of various kinds of offspring from a cross can be identified from knowledge of the kinds and proportions of gametes that parents can produce. • The probability of two independent events is the product of their separate probabilities. • When genes are linked, parental gametes will be produced and, in addition, recombinant gametes will arise from crossing over.
The closer two linked genes are, the less the chance that crossing over will separate them during meiosis. • The result of a test cross involving two genes can identify whether the genes are linked or not. • When genes are closely linked and separated by a known map distance, the results of a test cross can be predicted.
Summary • A pedigree uses symbols to show the pattern of appearance of an inherited trait across several generations. • Features of the pattern of inheritance of a trait may allow conclusions to be drawn regarding its mode of inheritance. • Common modes of inheritance are autosomal dominant, autosomalrecessive, X-linked dominant or X-linked recessive.
