What combinations of alleles could result from a crossover between bd and bd chromosomes?

Bb, Db , Bd and Dd

Explanation:

When a cross between two parents (having certain trait) is carried out , they share their alleles with each other to produce offspring with new traits. These offspring have gentically different allele combination as compared to their parents. A  genetic cross can be represented by a punnet square (as given below)

The cross between two parents with genotype "BD" and "bd" will result into the following offspring -

BD

bBbDb

dBdDd

The combination of allies that could result from crossover between BD and bd chromosomes are Bb, Db, Bd and Db.

In a situation where there is a cross between two parents of the same traits, their allies are shared with each other to produce offspring with new traits. Genetically, the offspring have a different allele combination which is different to that of their parent.

During prophase I of meiosis, crossing-over occurs and Homologous chromosomes exchange genetic material. This is primarily a form of recombination which increases genetic variability. For instance, a cross over between BD and bd chromosomes will produce Bb, Db, Bd and Db

There are three sources of genetic variation and crossing over is one of them. Other sources of genetic variation include random fertilization and independent assortment.  

Mendel’s law of independent assortment state that allele pairs separate during gametes formations. This implies that traits are transmitted to offspring to each other independently.

This principle was discovered when Mendel’s performed dihybrid crossing between plants of two traits such as pod color and seed color that are totally different from one another. After the plants self-pollinated, Mendel’s observed that the offspring has same ratio of 9:3:3:1 and because of this, Mendel came to a conclusion that the transmission of traits between the offspring were independently transmitted.  

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i think the answer is gold im not entirely sure