O

Family tree - standardized symbols

= males

= females

= affected individual (shading)

= deceased

= sex unknown

mating producing 2 males and 1 female consanguineous mating

FIGURE 1.19 Standard Symbols for a Family Tree located on the X-chromosome (but absent from the Y-chromosome). More males than females are color blind, as males have only one copy of this gene. If this gene is defective, they are affected. A female has two copies of the gene and only if both are defective will she be color blind. A variety of other hereditary diseases show sex linkage and their detrimental effects are therefore more commonly observed in males than females.

The chemical nature of genes—as segments of DNA— has major effects on their inheritance.

Neighboring Genes Are Linked during Inheritance

Although there is a random distribution of strands of DNA (chromosomes) during sexual reproduction, there is not always a random distribution of alleles. To illustrate this point we must remember that most higher organisms have tens of thousands of

FIGURE 1.20 Inheritance of a Sex-linked Gene

This family tree shows the inheritance of the wild-type ("A") and deleterious ("a") alleles of a gene that is carried on the X-chromosome. Since males have only one X-chromosome, they have only a single allele of this gene. The symbol "-" is used to indicate the absence of a gene. When the defective allele "a" is passed on to males, they will suffer its deleterious effects.

FIGURE 1.20 Inheritance of a Sex-linked Gene

This family tree shows the inheritance of the wild-type ("A") and deleterious ("a") alleles of a gene that is carried on the X-chromosome. Since males have only one X-chromosome, they have only a single allele of this gene. The symbol "-" is used to indicate the absence of a gene. When the defective allele "a" is passed on to males, they will suffer its deleterious effects.

A - a - A a a - A - A A

genes carried on multiple pairs of homologous chromosomes. Consider just a few of these genes—call them A, B, C, D, E, etc.—which have corresponding mutant alleles— a, b, c, d, e, etc. These genes may be on the same chromosome or they may be on different chromosomes. Let's assume that genes A, B and C are on one pair of homologous chromosomes and D and E are on a separate pair. Organisms that are heterozygous for all of these genes will have the genotype Aa, Bb, Cc, Dd, Ee. Consequently, A, B and C will be on one of a pair of homologous chromosomes and a, b, and c will be on the other member of the pair. A similar situation applies to D and E and d and e.

Alleles carried on different chromosomes are distributed at random among the offspring of a mating. For example, there is as much chance of allele d accompanying allele A during inheritance as allele D. In contrast, when genes are carried on the same chromosome, their alleles will not be distributed at random among the offspring. For example, because the three alleles A, B, and C are on the same chromosome, that is, the same molecule of DNA, they will tend to stay together. The same applies to a, b, and c. Such genes are said to be linked and the phenomenon is known as linkage (Fig. 1.21). Note that if two genes are very far apart they will not be linked in practice even if they reside on the same chromosome.

Recombination during Meiosis Ensures Genetic Diversity

However, the alleles A, B, and C (or a, b, and c) do not always stay together during reproduction. Crossing over occurs during the process of meiosis when the gametes are formed. First, the chromosome carrying a specific sequence of genes lines up next to the homologous chromosome with allele sites matching. Second, swapping of segments of the chromosomes can now occur by breaking and rejoining of the neighboring DNA strands. Note that the breaking and joining occurs in equivalent regions of the two chromosomes and neither chromosome gains or loses any genes overall. The genetic result of such crossing over, the shuffling of different alleles between the two members of a chromosomal pair, is called recombination or crossing over (Fig. 1.21). The farther apart two genes are on the chromosome, the more likely a crossover will form between them and the higher will be their frequency of recombination.

Genetic linkage is often defined, from a molecular viewpoint, as the tendency of alleles carried by the same DNA molecule to be inherited together. However, if two genes are very far apart on a very long DNA molecule, linkage may not be observed in practice. For example, consider a long chromosome, carrying genes A, B, C, D and crossing over When two different strands of DNA are broken and are then joined to one another linkage Two alleles are linked when they are inherited together more often than would be expected by chance, usually this is because they reside on the same DNA molecule (that is, on the same chromosome) recombination Mixing of genetic information from two chromosomes as a result of crossing over

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