TYPES OF GENETIC CROSSES
MONOHYBRID CROSS
TT x tt
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F1 generation Tt( tall) x Tt(tall)
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According to the law of dominance, there are two alleles of a gene, one of which is dominant over the other. The dominant gene expresses itself in one generation. As well, in the second generation.
A recessive gene is completely masked in the F1 generation. But in the F2 generation, it also expresses itself, showing that the gene is not lost in the F1 generation. It is only expressed in homozygous conditions.
A monohybrid cross is a backcross if an individual of the F1 generation is crossed with any of its parents.
A monohybrid cross can be in a test cross, where an individual of the F2 generation is crossed with a recessive parent. This cross determines homozygosity or heterozygosity of the individual. If the F2 individual is homozygous dominant, the test cross ratio will be 3:1; if the F2 individual is heterozygous, the test cross ratio will be 2:2.
Tt x tt
⇩
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Male/female gametes ⇩ |
T |
t |
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t |
Tt Tall |
tt short |
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t |
Tt Tall |
tt short |
DIHYBRID CROSS
A dihybrid cross is a cross to study two characters at a time. A dihybrid cross between a pea plant with green wrinkled seeds and yellow round seeds showed a ratio of 9:3:3:1.In the F1 generation, all the seeds were yellow and round, showing that yellow seed color and round seed shape are the dominant characters.
In the F2 generation, however, the parental type, as well as the recombinant type of seeds, are obtained. The parental types that were round yellow, both dominant characters, appeared in a 9/16 ratio. While the recombinant, i.e., green round 3/16, wrinkled yellow 1/16. This shows that the seed color and shape depend upon two different genes, which are inherited independently of each other in the next generation. All four possible types of gametes are formed. Each type of gamete has a 25% possibility. This is called the law of independent segregation, where genes segregate from each other independently. They combine in the next generation and result in the formation of parental types as well as recombinant individuals in the F2 generation.
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Male/ female gamete |
WG |
Wg |
wG |
wg |
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WG |
WWGG Yellow round |
WWGg Yellow round |
WwGG Yellow round |
WwGg Yellow round |
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Wg |
WWGg Yellow round |
WWgg Green round |
WwGg Yellow round |
Wwgg Green round |
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wG |
WwGG Yellow round |
WwGg Yellow round |
wwGG yellow wrinkled |
wwGg yellow wrinkled |
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wg |
WwGg Yellow round |
Wwgg Green round |
wwGg yellow wrinkled |
wwgg Green wrinkled |
The F2 generation ratio is 9:7
The complementary genes are those where both genes are required for the expression of a character. As seen in the flower color of Lathyrus odoratus, two genes control the expression of flower color. When both genes are present in dominant form, only then will purple flower color appear. But if a single gene is present in the dominant form, white flower color is formed. It may be understood from a two-step reaction where one gene controls the formation of the intermediate product, and the intermediate product is converted into purple flower color only in the presence of the product of the second gene. So, in the absence of any of these genes, the purple flower color is not formed.
Supplementary gene interaction
It is an interaction between two independent genes. Here, two independent genes, when present in the dominant form, together give a different phenotype; when these are present in the dominant form, they individually give a different phenotype.
As coat color in mice, AB together gives agouti, ab gives albino, AA, and BB gives black coat color.
It is the interaction between two different genes that have the same phenotypic effect. This interaction has an F2 generation phenotypic ratio of 15:1.
The fruit shape of Capsella bursa-pastoris (shepherd's purse). It has two types of fruit: triangular and oval. The shape of the fruit is determined by two independent genes present on different chromosomes. The dominant form of both genes produces a triangular fruit shape, while the recessive genes produce an oval shape.
AABB(Triangular) X aabb(Oval) ---> AaBb(Triangular) X AaBb----F1 Generation
This is against the law of complete dominance, where heterozygous individuals show an intermediate phenotype. In incomplete dominance, the genotypic ratio as well as the phenotypic ratios are the same. It can be studied in the flower color of Mirabilis jalapa. A homozygous dominant has red flower color heterozygous has pink flower color, and a homozygous recessive shows white flower color.
F2 generation has a phenotypic as well as genotypic ratio of 1:2:1.
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