How does a recessive trait show up as a visible trait?

Answer 1

Recessive alleles are only expressed in the phenotype if the organism is homozygous for the recessive allele (assuming diploidy). If the trait is sex-linked, then it will always show up in males if passed.

Answer 2

A recessive trait shows up as a visible trait when there are two recessive alleles for that trait. Since there are no dominant alleles for that individual, the recessive trait shows up.

Everyone has two versions of each gene, called alleles, stored in their genetic code: one from their father and one from their mother. The 'genotype' of an organism is the genetic sequence, the phenotype is how this genetic sequence is expressed in the organism for example, someone could have a genotype for hair colour as: 'Ginger allele' (g) and 'Brown allele' (B). If they had one pair of alleles (one from each parent) so they were either 'ginger ginger' (gg) or 'brown brown' (BB) they would express that gene physically in their phenotype this would be a 'pure' or 'true' breed known in Biology as 'homozygous'. If they have two different alleles for one quality as in the first example of a ginger and a brown allele (Bg) then the 'dominant' allele would be expressed, and as the ginger allele is 'recessive' to the brown allele the person would be brown haired - even though they are a 'carrier' for the ginger allele. A recessive allele (like the ginger allele) requires two recessive alleles to be expressed in the phenotype.

If a brown haired man and woman (Bg) were to have a child then there is a 1 in 4 chance the child would be ginger. This is because one allele must be inherited from both parents to control hair colour (as said before). The man would either donate a 'B' (brown) allele or a 'g' allele as would the mother. These would combine to form the child's genotype. The possible combinations are:

..................father

.................. B | g

mother B| BB | Bg

..............g| Bg | gg

As the table above shows there are three possible genotypes: 25% BB (brown), 50% Bg (brown) and 25% gg(ginger). This shows from two parents who are heterozygous - even though they don't express it themselves in their phenotype (they're both brown haired) they have a 25% chance of having a child that expressed the recessive trait.

Answer 3

Lethal alleles can be the result of a mutation. If the mutated lethal allele is recessive, then the lethal trait it codes for will only emerge if the allele is paired with another copy of itself.

Imagine a population that, through some fluke accident, has a disproportionately large number of organisms carrying such a lethal allele. The odds for any mating between individuals resulting in offspring carrying two copies of the allele are high. Many of these offspring will not survive into adulthood. Meanwhile, the few matings that result in offspring with just one copy or no copies of this allele *will* produce viable offspring. So in a few generations, the matings resulting in offspring with just one or no copies will outnumber the matings resulting in offspring with a lethal combination. Natural selection has occurred.

However, the shuffling of alleles is mostly random, and if a small portion of the population carries the allele, the odds are low that two individuals carrying a copy will mate to produce offspring that is homozygous for this allele, so the odds are that the allele will always remain present in the population.

Answer 4

because there are two recessive alleles for that trait.

Answer 5

A recessive trait only shows up if the individual is homozygous recessive for that trait.