Ampliaciones a la Herencia Mendeliana

Summary

This video explains key modifications and expansions to Mendelian genetics, including incomplete dominance, codominance, and multiple alleles. It covers phenomena like the Bombay blood group, lethal recessive alleles, and epistasis. Examples from both humans and animals are used, such as blood type inheritance and genetic traits in mice. The video also discusses the effect of different gene interactions on phenotypic proportions, like those involving the ABO blood group and albinism. Ultimately, it provides a comprehensive understanding of how genetic inheritance can deviate from classic Mendelian patterns.

Takeaways

• 😀 Incomplete dominance occurs when the offspring display a new phenotype that is a mixture of the parental traits, such as red and white flowers producing pink ones in the F1 generation.
• 😀 In incomplete dominance, heterozygotes show an intermediate phenotype, which is not a blend but a unique trait between the parental phenotypes.
• 😀 In complete dominance, both alleles express themselves fully in the heterozygote without blending, as seen in the MN blood group system.
• 😀 Multiple alleles can influence a trait, such as the ABO blood group system, where three alleles (A, B, O) produce multiple blood group phenotypes.
• 😀 The Bombay phenotype occurs when a mutation in the H gene prevents the addition of the necessary sugar to the blood antigen, masking other blood group phenotypes.
• 😀 Codominance is demonstrated when both alleles are equally expressed, such as in the case of human DNA markers or the ABO blood group system, where A and B alleles are both dominant over O.
• 😀 Lethal recessive alleles can result in death when present in a homozygous state, as shown by the yellow (Y) and agouti (A) genes in mice and the lethal effect in certain human genetic disorders.
• 😀 Human genetic diseases like Huntington's disease are caused by dominant alleles, where individuals show symptoms in later life (around 30-40 years).
• 😀 Epistasis occurs when one gene masks the expression of another gene, altering expected genetic ratios, as seen in blood group inheritance with the H gene masking other blood types.
• 😀 Genetic combinations involving epistasis can lead to altered ratios in offspring, such as the 9:3:3:1 ratio being changed when epistasis or co-dominance occurs in traits like pigmentation in mice or flower color in plants.

Q & A

What is incomplete dominance, and how is it observed in genetic inheritance?

Incomplete dominance is a genetic phenomenon where the offspring exhibit a new phenotype that is a blend of the parental traits. In this case, crossing red and white parental traits results in offspring with a mixture of both colors, showing neither red nor white dominance. This is seen in heterozygous individuals, and when they are crossed again, the parental phenotypes (red and white) reappear in the next generation.

How does Mendelian inheritance differ from the concept of incomplete dominance?

Mendelian inheritance involves dominant and recessive alleles where one allele completely dominates the other. In incomplete dominance, neither allele dominates, and the phenotype is an intermediate between the two parental traits, unlike the clear dominant-recessive pattern observed in Mendelian inheritance.

What is the genotype-phenotype ratio in incomplete dominance as seen in the F2 generation?

In the F2 generation of incomplete dominance, the genotype ratio is 1:2:1 (1 homozygous for one allele, 2 heterozygous, and 1 homozygous for the other allele). The phenotype ratio typically follows the same 1:2:1 pattern, where the heterozygotes show the blended intermediate phenotype.

Can you explain codominance and provide an example?

Codominance occurs when both alleles contribute equally and visibly to the organism's phenotype without blending. An example of codominance is the MN blood group system, where both M and N alleles are expressed independently in heterozygous individuals.

How do multiple alleles affect genetic inheritance? Provide an example.

Multiple alleles involve more than two variations of a gene, leading to more than two possible phenotypes. A classic example is the ABO blood group system, which has three alleles (A, B, O) that combine in different ways to form four possible phenotypes (A, B, AB, O).

What is the Bombay phenotype, and how does it differ from the ABO blood group system?

The Bombay phenotype is a genetic condition where individuals have a group O blood type despite possessing the A or B allele. This is due to a mutation that prevents the production of the H antigen, which is necessary for the proper functioning of the ABO blood group system. Even with A or B alleles, the lack of the H antigen results in the O blood group phenotype.

What is a lethal allele, and how can it affect offspring in a genetic cross?

A lethal allele is a recessive allele that, when present in a homozygous form, causes death, often before birth. For example, in mice, homozygous for the yellow coat color allele (which is recessive), the offspring do not survive, leading to altered genotype ratios in the offspring.

How does the concept of a lethal recessive allele apply to human genetics?

In humans, lethal recessive alleles can result in conditions such as achondroplasia, where homozygosity for the allele is lethal. Individuals who are heterozygous for the allele can survive, but homozygous individuals typically die early in development, affecting the inheritance patterns in the population.

What are epistasis and how does it influence gene expression?

Epistasis occurs when one gene masks or modifies the expression of another gene. In the case of blood types, for example, the H gene can mask the ABO blood group gene. Even if an individual carries the A or B allele, if they have two copies of the hh allele, they will express the O phenotype due to the lack of the H antigen.

How does epistasis alter expected genetic ratios in crosses?

Epistasis can change the expected genetic ratios by altering the expression of phenotypes. For example, in a cross involving epistasis, the standard 9:3:3:1 phenotypic ratio may be altered to a 1:2:1 or other ratios depending on the interaction between genes. The epistatic gene can mask the effects of other genes, leading to unexpected ratios.