In a certain species of pine trees, short needles (S) are dominant to long needles (s).
According to the Punnett square, what is the probability of an offspring having short needles?
both the parents used for the cross are homozygous dominant (SS), or
one is homozygous dominant (SS) and the otherheterozygous dominant (Ss), or
one is homozygous dominant (SS) and the other homozygous recessive (ss).
75%, if both the parents used for the cross are heterozygous dominant (Ss).
0%, if both the parents used for the cross are homozygous recessive (ss).
50%, if one of the parents used for the cross is heterozygous dominant and the other homozygous ressesive (Ss and ss).
Explanation:
The short needles (S) show the dominant phenotype.
The long needles (s) show the recessive phenotype.
Alleles are the contrasting forms of the same gene.
Genotype of an individual is the representation of its genetic constitution.
Phenotype of an individual is the characteristics that can be observed.
According to Mendel's Law of Dominance, of the two alleles in a diploid organism, one can show its effect even if a single copy of the allele is present. This is the dominant allele. The other allele can show its effect only when present in homozygous form. This is called the recessive allele.
When a crossing is done between two parents such that one shows the dominant phenotype (short needles), and has a homozygous dominant genotype (SS), then whatever be the genotype of the other parent, the offspring produced will show the dominant phenotype (short needles) and will have two possible genotypes (SS or Ss) homozygous dominant or heterozygous dominant, according to the Law of Dominance.
When a crossing is done such that both the parents are heterozygous dominant (Ss), the offspring possible are three having the short needles phenotype and one having long needle phenotype ( genotypically, one is homozygous dominant (SS), two are heterozygous dominant (Ss) and one is homozygous recessive(ss)). Therefore, 3 out of 4, that is, 75% individuals show the short needle phenotype.
When a crossing is done such that both the parents are homozygous recessive (ss), due to the absence of any dominant allele (S), no dominant short needle phenotype will be seen in the offspring.
When a crossing is done such that one of the parent is heterozygous dominant (Ss) and the other homozygousrecessive(ss), there are two types of offspring possible, one will have the heterozygous dominant genotype (Ss) (short needle phenotype) while the other will have the homozygous recessive genotype (ss) (long needle phenotype). Therefore, 1 out of 2, that 50% individuals show the short needle phenotype.
