Genetics: Monohybrid Cross
By Lhenn on Feb 25, 2010 with Comments 0
A cross between a tall plant and a short one is an example of a monohybrid cross (one factor cross) because it involves only one trait.
The alleles present in an organism constitute its genotype for that particular trait. Letters or descriptive phrases indicate a genotype. Since the original plants (P1) were pure-bred, they are said to be homozygous, meaning they have two identical alleles for the trait. As such, the tall plant contained two alleles for tallness (TT) and the short plant contained two alleles for shortness (tt). Genotype TT is called homozygous dominant, and genotype tt is called homozygous recessive.
During meiosis, the tall plant produced gametes containing the allele for tallness (T) and the short plant produced gametes containing the allele for shortness (t). after cross-pollination, all the offspring (F1) contained an allele for tallness and an allele for shortness (Tt). The genotype Tt is said to be heterozygous (having two different alleles for a trait). Since the allele for tallness (T) is dominant over the allele for shortness (t), the individuals in the F1 generation appeared tall. The outward appearance of an organism is called its phenotype.
In the next cross, the heterozygous individuals in the F1 generation were allowed to self-pollinate. One-half of the gametes produced by each parent contained the allele for tallness (T), and the other half contained the allele for shortness (t). During fertilization, random fusion of gametes occurred. Random fusion means that any male gamete has a chance of combining with any female gamete. The resulting F2 generation showed three different genotypes: TT, Tt, and tt. Comparing the number of offspring containing each genotype gives the genotypic ratio. In this case, the genotypic ratio (GR) of the F2 is 1TT : 2Tt : 1tt. All offspring with the dominant allele (T) will become tall, and all homozygous recessive offspring will become short. By simple addition, ¾ or 75% of the plants will become tall (1/4 TT + 2/4 Tt) and ¼ or 25% will become short. Comparing the number of offspring showing each phenotype gives the phenotypic ratio. In this case, the phenotypic ratio is 3 tall : 1 short.
Solving Monohybrid Crosses
1. First, one has to know which characteristic is dominant. Geneticists have identified certain traits that exhibit dominance in plants, animals, and humans. For example, in humans, curly hair is dominant over straight hair. Therefore, the allele for curly hair can be represented by the uppercase letter C, and the allele for straight, the lowercase letter c.
Key: C = curly hair
c = straight hair
2. Based on the key, determine the genotypes of the parents. Consider this problem:
a. If a man, homozygous for curly hair reproduces with his wife who has straight hair, what type of hair will their children have?
Since the man is homozygous curly hair, his genotype consists of two alleles for curly. Thus, you can write the man’s genotype as CC. On the other hand, since the woman exhibits the recessive characteristic, which is straight, her genotype is cc. Note that recessive characteristics are always homozygous.
3. Identify the gametes produced by each parent and perform the monohybrid cross by random combination of gametes. Since the parents are both homozygous, each produces only one kind of gamete. CC will form gametes with the C allele, and cc will produce gametes with the c allele. These gametes combine during fertilization and produce an offspring with a heterozygous genotype (Cc).
4. Interpret the result of the cross. The genotype of the offspring resulting from the cross is heterozygous. Since there is only one possible genotype in the offspring, we can state the genotypic ratio (GR) as GR:100% Cc. Since the allele for curly (C) is dominant over the allele for straight (c), then the offspring is phenotypically curly. The phenotypic ratio can be stated as PR:100% curly.
5. Answer the question asked in the problem. In this case, the answer to the question is: Their offspring will have a curly hair.
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