Chapter 7

What are autosomal genes?

Autosomal genes are genes located on the autosomes (non-sex chromosomes). Both males and females have these genes in two copies, one on each homologous chromosome, influencing traits like hair texture.

How do autosomal genes affect an organism's phenotype?

Each individual has two alleles of every autosomal gene. These alleles interact to determine the organism’s phenotype, with dominant alleles potentially masking recessive alleles in heterozygous genotypes.

What is the difference between autosomes and sex chromosomes?

• Autosomes are chromosomes not directly involved in determining sex, appearing in pairs with identical sets of genes in males and females.

• Sex chromosomes (X and Y in humans) determine the sex of the organism and carry different sets of genes.

How are autosomal recessive genetic disorders inherited?

• Autosomal recessive disorders require two copies of the recessive allele for the trait to be expressed. Individuals with only one recessive allele are carriers who do not show symptoms but can pass the allele to offspring.

What is a carrier, and how does it relate to autosomal recessive disorders?

A carrier is an individual who has one copy of a recessive allele for a trait but does not express the disorder’s phenotype. Carriers can still pass the allele on to their offspring, who may exhibit the disorder if they inherit two recessive alleles.

How are autosomal dominant genetic disorders inherited?

Autosomal dominant disorders occur if at least one dominant allele is present. Individuals with a single mutated dominant allele will show the disorder, allowing it to be passed to the next generation.

What are sex-linked genes?

Sex-linked genes are genes located on sex chromosomes (X or Y). In humans, most sex-linked genes are found on the X chromosome because it has more genes than the Y chromosome.

How do sex-linked gene expressions differ between males and females?

• Males (XY) express all alleles on their single X chromosome because there’s no second X to mask recessive traits.

• Females (XX) can be carriers for recessive traits on the X chromosome without expressing them, as they have two X chromosomes.

Why are males more likely than females to have certain sex-linked disorders?

Males have only one X chromosome, so if they inherit a recessive allele on the X chromosome, they will express the disorder. Females must inherit two copies of the recessive allele (one on each X chromosome) to express the same disorder.

What is X chromosome inactivation, and why does it occur in female mammals?

X chromosome inactivation is a process where one of the two X chromosomes in each female cell randomly becomes inactive. This balances the expression of genes on the X chromosome between males (who have one X) and females (who have two Xs).

How do autosomal genes differ from sex-linked genes in determining traits?

• Autosomal genes influence traits that are not related to an organism’s sex and typically require two alleles to express a phenotype, while sex-linked genes are located on sex chromosomes and their expression can depend on the organism’s sex.

Why do female carriers of a recessive sex-linked disorder typically not show the disorder’s phenotype?

Female carriers have two X chromosomes. While one may carry the recessive allele, the dominant allele on the other X chromosome often masks the recessive allele’s effect, preventing the female from expressing the disorder.

How can phenotype depend on interactions of alleles?

• Phenotype can be influenced by different patterns of allele interactions such as incomplete dominance, codominance, polygenic traits, and epistasis, each affecting how traits are expressed in an organism.

What is incomplete dominance, and how does it affect phenotype?

Incomplete dominance occurs when a heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, crossing a red-flowered plant with a white-flowered plant yields offspring with pink flowers.

What is codominance, and how does it differ from incomplete dominance?

• Codominance occurs when two different alleles are fully expressed at the same time, creating a phenotype that shows both traits distinctly. In incomplete dominance, the heterozygous phenotype is a mixture; in codominance, both alleles contribute equally and separately.

What are polygenic traits, and how do they affect phenotype?

Polygenic traits are characteristics influenced by multiple genes. Each gene may have two or more alleles, and their combined expression results in a wide range of phenotypes. Examples include human skin color, which is affected by several genes.

What is an epistatic gene, and how does it influence phenotype?

• An epistatic gene is a gene that can interfere with the expression of other genes. It can overshadow all other genes involved in a particular trait, effectively determining whether or not those genes are expressed.

How do multiple-allele traits differ from polygenic traits?

• Multiple-allele traits involve genes that have more than two allele forms within a population (e.g., ABO blood types in humans).

• Polygenic traits involve multiple genes, each potentially having multiple alleles, working together to produce a single phenotype (e.g., human skin color).

Summarize how incomplete dominance and codominance alter phenotypic ratios in offspring.

• Incomplete dominance results in a phenotype that blends parental traits, often creating a 1:2:1 ratio of phenotypes.

• Codominance leads to offspring expressing both parental phenotypes distinctly, as seen in the AB blood type, where both A and B antigens are fully expressed.

What is gene linkage?

Gene linkage occurs when genes are located close together on the same chromosome, causing them to be inherited together rather than assorted independently.

What are linked genes, and how do they affect inheritance patterns?

Linked genes are genes located on the same chromosome and tend to be inherited together. They do not follow Mendel's law of independent assortment and can change inheritance ratios because they move as a unit during meiosis unless crossing over separates them.

If linked genes do not always appear to be inherited together, what process can separate them?

• Crossing over during meiosis can separate linked genes by exchanging chromosome segments, creating new allele combinations and reducing how often these genes are inherited together.

What is a linkage map, and what does it represent?

A linkage map is a diagram that shows the relative locations of genes on a chromosome. It represents how frequently crossing over occurs between genes, giving an estimate of their distances from each other on the chromosome.

How did William Bateson and R.C. Punnett contribute to our understanding of gene linkage?

Bateson and Punnett studied dihybrid crosses in pea plants. They observed that some traits did not follow Mendel's expected 9:3:3:1 ratio, suggesting certain genes are inherited together, laying the foundation for understanding gene linkage.

How did Thomas Hunt Morgan’s research with fruit flies advance the concept of gene linkage?

Morgan observed that fruit fly traits that did not assort independently grouped into linked traits. By studying these patterns, he concluded that linked genes were located on the same chromosome and that chromosomes (not individual genes) assort independently during meiosis.

Why do some traits follow Mendel’s ratios for independent assortment while others do not?

• Traits determined by genes on separate chromosomes or far apart on the same chromosome typically follow Mendel’s independent assortment ratios. When genes are close together on the same chromosome (linked genes), they are inherited together more often and do not follow those ratios.

What are map units on a linkage map?

• Map units measure the distance between genes on a chromosome. One map unit corresponds to a 1% chance of recombination (crossing over) occurring between two genes during meiosis.

Summarize how a linkage map is created from observations of linked traits.

To create a linkage map:

1. Determine how frequently different pairs of traits are inherited together.

2. Use the recombination frequencies (derived from crossing over percentages) to estimate the distances between genes.

3. Arrange genes on the chromosome in the order that best explains their observed recombination frequencies.

How do the patterns of inheritance in humans compare to those in pea plants and fruit flies?

Humans follow the same basic genetic principles as pea plants and fruit flies. Chromosomes assort independently during meiosis, and allele interactions such as dominance, recessiveness, and more complex patterns apply to human traits as well.

Why can the inheritance of human traits be more complex than simple Mendelian genetics?

Many human traits are influenced by multiple genes (polygenic traits), have incomplete dominance or codominance, and can be affected by environmental factors. This complexity means that few human traits follow a straightforward dominant-recessive pattern.

What are some examples of single-gene traits in humans?

Traits such as having a widow’s peak hairline or attached earlobes are single-gene traits following simple dominant and recessive patterns. Widows’ peak is dominant, while a straight hairline is recessive.

• What is the difference between autosomal disorders and sex-linked disorders in humans?

• Autosomal disorders are caused by genes on autosomes and affect both males and females equally.

• Sex-linked disorders are caused by genes on sex chromosomes (usually the X chromosome) and often affect males more frequently due to having only one copy of the X chromosome.

Can females be carriers of genetic disorders?

• Autosomal carrier: Both males and females can be carriers for recessive autosomal disorders without expressing symptoms.

• Sex-linked carrier: Only females can be carriers for recessive sex-linked disorders, as they have two X chromosomes. Males either express the disorder or do not carry the gene.

What is a pedigree, and what information does it provide?

A pedigree is a chart that traces the inheritance of a particular trait through several generations in a family. It shows relationships among family members, which individuals express the trait, and which may be carriers.

How are individuals and traits represented in a pedigree?

• In a pedigree:

  • Squares represent males.

  • Circles represent females.

  • Shaded shapes indicate individuals who express the trait.

  • Half-shaded shapes indicate carriers.

  • Clear shapes indicate individuals who do not express the trait and are not carriers.

How can pedigrees be used to determine if a trait is autosomal or sex-linked?

In a pedigree:

• If a trait appears equally in males and females, it is likely autosomal.

• If the trait is more common in males and tends to skip generations, it is likely sex-linked, meaning the gene is on the X chromosome.

What is a karyotype, and how is it useful in genetics?

A karyotype is an image of all the chromosomes in a cell, used to identify chromosome abnormalities. It shows chromosome pairs arranged by size and can help detect additions, deletions, or errors such as trisomy (an extra chromosome).

What kinds of changes can a karyotype reveal?

Karyotypes can reveal:

• Changes in chromosome number, such as Down syndrome (an extra copy of chromosome 21).

• Changes in chromosome structure, such as deletions or duplications of chromosome segments.

• Why must a combination of methods be used to study human genetics, and what are some of these methods?

Human genetics is complex due to large genome size and ethical considerations. Researchers use:

• Pedigrees to study inheritance patterns.

• Karyotypes to visualize chromosome abnormalities.

• Molecular genetics (DNA testing and the Human Genome Project) to study genes directly.