29. Heredity

Dominant-recessive inheritance, incomplete dominance, and codominance

Patterns of Inheritance

Mendel's Law of Dominance

Traits that are associated with the X or Y chromosomes (sex chromosomes) exhibit sex-linked inheritance. Red-green colorblindness is inherited as a sex-linked trait. Blood type is determined by several different alleles, making this a trait that exhibits multiple-allele inheritance. The alleles for blood type exhibit what is called codominance. This means that each inherited allele will equally contribute to the phenotype of the organism. Polygene inheritance depends upon multiple gene pairs at different locations acting in tandem that have a "dose" or additive effect on the phenotype. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. Traits that are sex-linked or autosomal can exhibit a dominant-recessive relationship.

 Red-green color blindness exhibits __________ inheritance. 

MyReadinessTest Video: Allele Interactions

Incomplete Dominance and Codominance 

 Sometimes, one allele masks or suppresses the expression of its partner. Such an allele is said to be dominant, whereas the allele that is masked is said to be recessive. 

 An allele that is able to mask the expression of its partner allele is said to be __________. 

 Some examples of recessive inheritance are the more desirable genetic condition; for example, normal vision is due to recessive alleles, but astigmatism is a dominant trait. However, most genetic disorders are inherited as simple recessive traits. These include conditions as different as albinism, a lack of skin pigmentation; cystic fibrosis, a condition of excessive mucus production that impairs lung and pancreatic functioning; and Tay-Sachs disease, a disorder of brain lipid metabolism caused by an enzyme deficit that shows itself a few months after birth. 

 Many genetic disorders, such as albinism, cystic fibrosis, and Tay-Sachs disease, are inherited as simple recessive traits. 

 Heterozygous parents who have had one child with a recessive disease have a 25% chance of having a second child born with the same recessive disease. 

 Heterozygous parents who have had one child with a recessive disease have a __________% chance of having a second child born with the same recessive disease. 

X-linked Inheritance and Punnett Squares

 In incomplete dominance, the heterozygous state is expressed as an intermediate between both homozygous states. 

 The form of inheritance in which the heterozygous state is expressed as an intermediate is __________. 

 the gene for brown hair is carried on the maternally derived chromosome 

 By convention, a dominant allele is represented by a capital letter. 

 the gene for brown hair is carried on the paternally derived chromosome 

If the allele for brown hair is represented by B, it means that __________.

Dominant Alleles vs Recessive Alleles 

 A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents with known genotypes. The Punnett square predicts only the probability of a particular genotype (and phenotype). The larger the number of offspring, the greater the likelihood that the observed ratios will conform to the predicted values. 

 The larger the number of offspring, the greater the likelihood that the observed ratios will conform to the values predicted by the Punnett square. 

Sex Chromosomes and Sex-Linked Traits

 it will only be expressed 25% of the time 

 Color blindness is carried on the X chromosome, and males always express X-linked genes. 

 it will be expressed only if two copies are present 

 If a male inherits a sex-linked gene for color blindness, __________. 

Punnett Squares and Sex-Linked Traits

Polygenic Inheritance

Although we inherit only two alleles for each gene, some genes exhibit more than two allele forms, leading to a phenomenon called multiple-allele inheritance. For example, three alleles determine the ABO blood types in humans: IA, IB, and i. Each of us receives two of these. The IA and IB alleles are codominant, and both are expressed when present, resulting in the AB blood type. The i allele is recessive to the other two alleles.

 Codominant alleles are present in __________. 

 The A blood type allele is dominant and the O blood type allele is recessive. In the heterozygous state, the O blood type allele is not phenotypically expressed. 

 The blood type cannot be determined from the given information. 

 A person who inherits the A and the O blood type alleles will possess which blood type? 

Multiple Alleles and Codominance

 Genes on the same chromosome are said to be linked because they are transmitted as a unit to daughter cells during mitosis. The linkage of genes does not determine the dominant-recessive pattern of their inheritance. 

How to Draw a Punnett Square 

 Down syndrome is a chromosome abnormality due to nondisjunction. 

 Which of the following disorders is NOT inherited as a simple recessive trait? 

The number of different gamete types resulting from independent assortment of homologous chromosomes during meiosis I can be calculated for any genome from the formula 2n, where n is the number of homologous pairs. For five pairs of homologous chromosomes, it would be 2 &times; 2 &times; 2 &times; 2 &times; 2 = 32.

 An organism with five pairs of homologous chromosomes could produce __________ different gametes on the basis of independent assortment alone. 

 Very few traits are traced to a single gene. 

 While nutrition during infancy can affect the phenotype of an individual, it does not change the original genotype. 

 A single allele reflects a single gene pair, and very few human traits are determined by a single pair of genes. 

 Most human traits are determined by __________. 

 The X and Y sex chromosomes are not homologous in the true sense. The Y, which contains the gene that determines maleness, is much smaller than the X chromosome. The X bears over 1400 genes, and a disproportionately large number of them provide code for proteins important to brain function. Because the Y carries only 200 genes, it lacks many of the genes present on the X. 

 The X and Y sex chromosomes are the best examples of homologous chromosomes. 

Co-dominance and Incomplete Dominance 

According to the assumptions in the problem, in order for a child to have blue eyes he or she must have a bb genotype. A Punnett square analysis would predict that 50% of the offspring from this couple would have the bb genotype and thereby express the recessive blue eye trait.

Use the following information to solve the problem using a Punnett square: Assume that the dominant B allele encodes brown eyes and the recessive b allele encodes blue eyes. An individual with the genotype bb has children with an individual of the genotype Bb. What is the percent chance that they would have children with blue eyes?

 In the human blood type AB, the alleles are codominant.  

 In the human blood type AB, both alleles are expressed and are therefore __________. 

 There are 46 chromosomes in a normal human diploid cell. There are 23 chromosomes in a haploid human cell. Gametes (egg and sperm cells) are haploid cells. 

 How many chromosomes are in a normal haploid human cell? 

 Traits that display continuous phenotypic variation are usually determined by polygene inheritance.  

 Traits that display continuous phenotypic variation are usually determined by which form of inheritance? 

 they have fewer chromosomes than females 

 Males tend to express X-linked conditions more often than females because any defect in the X chromosome (from the mother) has no counterpart on the Y chromosome. 

 they have more chromosomes than females 

 there is no corresponding allele on their Y chromosomes 

 X-linked recessive traits are expressed more frequently in males because __________. 

 Dominant alleles will be expressed in the heterozygous state. 

 In dominant-recessive inheritance, if an individual is heterozygous, which phenotype will be expressed? 

 Each parent has a 50% chance of donating the recessive allele to the baby. 50% &times; 50% = 25% 

 What is the probability that a child will be born with a recessive trait if both parents are heterozygous for the trait? 

Incomplete Dominance

 Extrachromosomal inheritance involves genes passed on by the mother’s mitochondria. Mutant forms of mitochondrial genes cause some rare genetic diseases. 

 Extrachromosomal inheritance involves genes passed on by the mother’s __________. 

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