34 Chapter 2 • Patterns of Inheritance The concept of equal segregation has been given formal recognition as Mendel’s frst law: The two mem-bers of a gene pair segregate from each other into the gametes; so half the gametes carry one member of the pair and the other half of the gametes carry the other member of the pair. Now we need to introduce some more terms. The individuals represented as A / a are called heterozygotes or, sometimes, hybrids, whereas the individuals in pure lines are called homozygotes. In such words, hetero-means “different” and homo- means “identical.” Thus, an A / A plant is said to be homozygous dominant; an a / a plant is homozygous for the recessive allele, or homozy-gous recessive. As stated in Chapter 1, the designated genetic constitution of the character or characters under study is called the genotype. Thus, Y / Y and Y / y , for ex-ample, are different genotypes even though the seeds of both types are of the same phenotype (that is, yellow). In such a situation, the phenotype is viewed simply as the outward manifestation of the underlying genotype. Note that, underlying the 3:1 phenotypic ratio in the
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.