Heredity and Genetics
5.1 Explain Mendel's laws of inheritance and the conditions for Mendel's first and
second laws.
Mendel’s first law of inheritance: Each of the gene variants in the gene pair segregates
randomly and therefore has an equal probability of being passed on to the next generation.
Mendel’s second law of inheritance: Different gene pairs assort to the next generation
independently of one another. The second law applies only to genes located on different
chromosomes. The traits that Mendel selected are known today to be located on different
chromosomes, allowing them to be inherited independently of each other. Most hereditary
diseases and traits do not follow Mendelian inheritance. A trait is often governed by a complex
interplay between many genes and environmental factors.
5.2 Explain the inheritance pattern of a single trait controlled by a gene pair, including:
• Complete dominance
• Codominance
• Incomplete dominance
Dominant and recessive
In complete dominance and recessive inheritance, the dominance relationship is absolute; one
gene variant is dominant and determining for the phenotype. The recessive phenotype
manifests only if both gene variants are recessive.
Codominance
Codominance expresses both gene variants equally, and both variants contribute to the
phenotype. When both gene variants in a gene pair exhibit their full effect simultaneously, we
have codominance. Here, there are no intermediate forms, as occurs when the dominance
relationship between the gene variants is incomplete. An example is the blood type system.
Blood type AB expresses both gene variants that give rise to antigen A and B equally,
exemplifying codominance. In codominance, the phenotype features traits from both
homozygous parents in the P generation. The dominance relationship does not indicate which
gene variants are more common.
Incomplete dominance
The dominance relationship between gene variants in a gene pair can be incomplete.
Incomplete dominance is a middle ground between the extremes of complete dominance and
recessive inheritance, where the heterozygous offspring do not have the same phenotype as
either of the homozygous parents. An example is the crossing of red and white flowers,
resulting in pink flowers: F^R crossed with F^H produces F^RF^H, where neither gene variant
is dominant over the other, yielding pink flowers. In incomplete dominance or intermediate
inheritance, we can obtain phenotype ratios of 1:2:1, distinguishing it from dominant
inheritance, where phenotypes must be in a 3:1 or 1:1 ratio.
, Inheritance with multiple gene variants:
When a gene pair has more than two possible variants, these are called multiple gene variants.
For example, in the blood type system, the trait is controlled by a single gene pair . We would
expect to see A, B, and AB due to codominance. However, there are four distinct ABO blood
types, resulting from three different gene variants that can comprise the gene pair: Ia, Ib, and
i. Ia determines that antigen A is present on the blood cells, Ib determines the same for B, while
i has no effect on which antigens will be on the blood cells. Ia and Ib are codominant to each
other and dominant over i. A person with genotype ii has neither antigen A nor B on the surface
and is classified as blood type O.
5.2 Create Punnett squares for test crosses
A test cross is used when we want to breed for specific traits, making it important to know the
genotype of the parents. A test cross typically involves crossing with a known genotype. We
can examine inheritance patterns in a Punnett square, showing the probability of the
offspring's genotype when two individuals are crossed—it illustrates possible genotypes in a
crossing experiment.
5.2 Provide examples and explain the inheritance patterns for some common
hereditary diseases:
• Recessive hereditary diseases
• Dominant hereditary diseases
Recessive:
Recessive hereditary diseases are often linked to mutations that result in non-functional
proteins. If the mutation affects a gene coding for an enzyme, the effect of the enzyme is
hindered. However, our cells have two variants of a gene; if one variant is mutated, the healthy
variant can produce functional proteins to carry out the protein's function in the cell,
compensating for the disease allele. Therefore, the inheritance of the disease is recessive, and
affected individuals have a homozygous genotype for the recessive disease allele. Still, one can
be a carrier of the disease without being affected and may pass the disease allele to the next
generation. Healthy parents can consequently have affected children if both are carriers. An
example of a recessive hereditary disease is cystic fibrosis.
Dominant:
For diseases that follow a dominant inheritance pattern, it is sufficient to have one copy of the
disease allele to be affected. A healthy person must be homozygous for the normal gene
variant. Dominant disease alleles causing a fatal disease are much rarer than fatal recessive
diseases. If a dominant disease gene causes the child to die before it has a chance to
reproduce, the disease allele will not be passed on to new generations. Thus, dominant disease
alleles can only be inherited if they lead to death at a relatively older age, allowing the disease
gene to potentially be passed to the next generation. In contrast, lethal recessive disease
genes can be passed from generation to generation by heterozygous carriers with a healthy
phenotype. An example of a dominant hereditary disease is Huntington's disease.
5.2 Explain the inheritance patterns for both recessive and dominant diseases using
pedigrees.
5.1 Explain Mendel's laws of inheritance and the conditions for Mendel's first and
second laws.
Mendel’s first law of inheritance: Each of the gene variants in the gene pair segregates
randomly and therefore has an equal probability of being passed on to the next generation.
Mendel’s second law of inheritance: Different gene pairs assort to the next generation
independently of one another. The second law applies only to genes located on different
chromosomes. The traits that Mendel selected are known today to be located on different
chromosomes, allowing them to be inherited independently of each other. Most hereditary
diseases and traits do not follow Mendelian inheritance. A trait is often governed by a complex
interplay between many genes and environmental factors.
5.2 Explain the inheritance pattern of a single trait controlled by a gene pair, including:
• Complete dominance
• Codominance
• Incomplete dominance
Dominant and recessive
In complete dominance and recessive inheritance, the dominance relationship is absolute; one
gene variant is dominant and determining for the phenotype. The recessive phenotype
manifests only if both gene variants are recessive.
Codominance
Codominance expresses both gene variants equally, and both variants contribute to the
phenotype. When both gene variants in a gene pair exhibit their full effect simultaneously, we
have codominance. Here, there are no intermediate forms, as occurs when the dominance
relationship between the gene variants is incomplete. An example is the blood type system.
Blood type AB expresses both gene variants that give rise to antigen A and B equally,
exemplifying codominance. In codominance, the phenotype features traits from both
homozygous parents in the P generation. The dominance relationship does not indicate which
gene variants are more common.
Incomplete dominance
The dominance relationship between gene variants in a gene pair can be incomplete.
Incomplete dominance is a middle ground between the extremes of complete dominance and
recessive inheritance, where the heterozygous offspring do not have the same phenotype as
either of the homozygous parents. An example is the crossing of red and white flowers,
resulting in pink flowers: F^R crossed with F^H produces F^RF^H, where neither gene variant
is dominant over the other, yielding pink flowers. In incomplete dominance or intermediate
inheritance, we can obtain phenotype ratios of 1:2:1, distinguishing it from dominant
inheritance, where phenotypes must be in a 3:1 or 1:1 ratio.
, Inheritance with multiple gene variants:
When a gene pair has more than two possible variants, these are called multiple gene variants.
For example, in the blood type system, the trait is controlled by a single gene pair . We would
expect to see A, B, and AB due to codominance. However, there are four distinct ABO blood
types, resulting from three different gene variants that can comprise the gene pair: Ia, Ib, and
i. Ia determines that antigen A is present on the blood cells, Ib determines the same for B, while
i has no effect on which antigens will be on the blood cells. Ia and Ib are codominant to each
other and dominant over i. A person with genotype ii has neither antigen A nor B on the surface
and is classified as blood type O.
5.2 Create Punnett squares for test crosses
A test cross is used when we want to breed for specific traits, making it important to know the
genotype of the parents. A test cross typically involves crossing with a known genotype. We
can examine inheritance patterns in a Punnett square, showing the probability of the
offspring's genotype when two individuals are crossed—it illustrates possible genotypes in a
crossing experiment.
5.2 Provide examples and explain the inheritance patterns for some common
hereditary diseases:
• Recessive hereditary diseases
• Dominant hereditary diseases
Recessive:
Recessive hereditary diseases are often linked to mutations that result in non-functional
proteins. If the mutation affects a gene coding for an enzyme, the effect of the enzyme is
hindered. However, our cells have two variants of a gene; if one variant is mutated, the healthy
variant can produce functional proteins to carry out the protein's function in the cell,
compensating for the disease allele. Therefore, the inheritance of the disease is recessive, and
affected individuals have a homozygous genotype for the recessive disease allele. Still, one can
be a carrier of the disease without being affected and may pass the disease allele to the next
generation. Healthy parents can consequently have affected children if both are carriers. An
example of a recessive hereditary disease is cystic fibrosis.
Dominant:
For diseases that follow a dominant inheritance pattern, it is sufficient to have one copy of the
disease allele to be affected. A healthy person must be homozygous for the normal gene
variant. Dominant disease alleles causing a fatal disease are much rarer than fatal recessive
diseases. If a dominant disease gene causes the child to die before it has a chance to
reproduce, the disease allele will not be passed on to new generations. Thus, dominant disease
alleles can only be inherited if they lead to death at a relatively older age, allowing the disease
gene to potentially be passed to the next generation. In contrast, lethal recessive disease
genes can be passed from generation to generation by heterozygous carriers with a healthy
phenotype. An example of a dominant hereditary disease is Huntington's disease.
5.2 Explain the inheritance patterns for both recessive and dominant diseases using
pedigrees.
