D3.2 Inheritance

What is the basis of inheritance?

Parents pass genes to offspring in gametes.

What is the chromosome number of gametes?

Haploid—they contain one chromosome of each type.

What is the chromosome number in a zygote?

Diploid—it contains two chromosomes of each type.

How is the chromosome number halved in parents to produce gametes?

Through meiosis.

How many haploid nuclei are produced during meiosis?

Four.

What does a diploid nucleus contain?

Two copies of each gene.

What does a haploid nucleus contain?

Only one copy of each gene.

What is the method used to investigate patterns of inheritance in plants?

Crossing varieties of plants, such as pea plants.

How is pollen transferred in controlled cross-pollination?

Using a paintbrush or directly dabbing an anther with pollen onto the stigma.

What is self-pollination?

Transfer of pollen from an anther to a stigma of the same plant. What is self-fertilization?

What are the geneticist terms for the parent and first offspring generations?

P generation and F1 (first filial) generation.

What traits did Mendel study in pea plants?

Seven traits, each with two forms (e.g., tall/dwarf, smooth/wrinkled seeds).

What is a Punnett grid used for?

Analyzing the results of genetic crosses.

What is an allele?

A version of a gene.

How are new alleles generated?

By mutation.

What is the genotype of an individual with two identical alleles of a gene?

Homozygous.

What is the genotype of an individual with two different alleles of a gene?

Heterozygous.

What does the phenotype of an organism represent?

Its observable traits or characteristics.

What factors influence most phenotypic traits?

The interaction between genotype and environment.

What happens when a heterozygous plant self-pollinates?

Four possible outcomes, shown in a Punnett grid.

What is phenotypic plasticity?

The ability of an organism to vary its traits in response to the environment.

What causes phenylketonuria (PKU)?

A recessive allele of the gene coding for phenylalanine hydroxylase.

What are the effects of excess phenylalanine in PKU?

Impaired brain development and intellectual disability.

How can PKU be managed?

By a diet low in phenylalanine.

What is a gene pool?

All the genes of all individuals in a population.

What is evolution?

Changes in the gene pool over time.

What is a single-nucleotide polymorphism (SNP)?

A position in a gene where different bases can occur.

What is an example of a gene with multiple alleles in apples?

The S-gene.

What is the role of the S-gene in apples?

Prevents self-pollination and reduces inbreeding.

What is an example of multiple alleles in humans?

The ABO blood group system.

What are the alleles of the ABO blood group gene?

IA, IB, and i.

What phenotype does the genotype IAIB produce?

Blood group AB.

What type of dominance is shown in the ABO blood group system?

Codominance.

What is an example of incomplete dominance?

Pink flowers from a cross between red and white Mirabilis jalapa plants.

What is a monohybrid cross?

A genetic cross involving one character or gene.

What are F1 hybrids?

Offspring of two pure-breeding parents with different alleles.

What is the probability of two carriers having a child with a recessive genetic disease?

25%.

What is a carrier?

An individual with one recessive and one dominant allele who does not show symptoms of the disease.

What enzyme does the PKU gene code for?

Phenylalanine hydroxylase.

How are blood transfusion complications prevented?

By matching the patient’s blood group with the donor’s.

What do IA and IB alleles cause in blood cells?

Modifications to the glycoprotein on red blood cells.

What is blending inheritance?

The disproven idea that offspring traits are intermediate between parents.

What ratio did Mendel observe in the F2 generation of tall and dwarf pea plants?

3:1.

Why is a mutation in a recessive allele less likely to affect phenotype?

A functioning dominant allele can compensate.

What does codominance result in?

A dual phenotype, as seen in blood group AB.

How is phenotypic plasticity different from evolution?

It is reversible and involves changes in gene expression, not alleles.

What is the main cause of genetic diseases?

Recessive alleles of genes.

What is the genetic makeup of a pure-breeding organism?

Homozygous for all genes.

What do Punnett grids help calculate?

The ratio of phenotypes and genotypes in offspring.

How do dominant alleles affect polypeptide function?

They allow normal polypeptide production even if a recessive allele is present.

What does incomplete dominance result in?

A phenotype intermediate between two parents, like pink flowers.

What is codominance?

When both alleles are expressed equally in the phenotype.

What happens when male and female gametes fuse?

Their nuclei join together, doubling the chromosome number, and the nucleus of the zygote becomes diploid.

How do parents produce gametes?

Parents halve the chromosome number of their body cells from diploid to haploid through meiosis.

What happens during meiosis?

A diploid nucleus divides twice to produce four haploid nuclei, each containing only one copy of each gene.

How can patterns of inheritance be investigated?

Patterns of inheritance can be investigated by crossing varieties of pea or other flowering plants.

How is pollen transferred in plant crosses?

Pollen is transferred from the anthers of one plant (male parent) to the stigmas of another plant (female parent).

How can pollen transfer be performed manually?

A paintbrush can transfer pollen, or an anther with pollen can be dabbed directly onto the stigma.

How are other sources of pollen prevented from reaching the stigma?

By cutting off all the anthers of the same flower before their pollen matures and enclosing the flower in a paper bag.

What is self-pollination, and what does it result in?

Self-pollination is the transfer of pollen from an anther to a stigma on the same plant, resulting in self-fertilization.

What happens when pollen germinates?

Male gametes are carried down to the ovary in a pollen tube, where they fuse with female gametes to form zygotes.

What happens to each zygote?

Each zygote develops into an embryo inside a seed.

How do alleles differ?

Alleles may differ by as little as one base in the base sequence or by large sections.

How are new alleles of a gene generated?

New alleles are generated by mutation.

How many alleles of most genes do humans and other diploid organisms have?

Humans and other diploid organisms have two alleles of most genes, one from each parent.

What is the genotype of an organism?

The genotype is the combination of alleles, such as DD, dd, or Dd.

What are homozygous individuals?

Individuals with genotypes DD or dd are homozygous because all their gametes have the same allele.

What are heterozygous individuals?

Individuals with the genotype Dd are heterozygous because they produce gametes with different alleles of the gene.

What is the phenotype of an organism?

The phenotype is the organism’s observable traits or characteristics.

What factors influence phenotypic traits?

Most phenotypic traits result from the interaction between genotype and environment, although some are solely determined by one or the other.

What does incomplete dominance result in?

Pink flowers, an intermediate phenotype, as seen in Mirabilis jalapa.

How is the expected ratio of flower colors from a cross between two pink-flowered plants determined?

Using a Punnett grid.

What determines sex in humans?

The 23rd pair of chromosomes.

What are the two types of human sex chromosomes?

The X chromosome and the Y chromosome.

How many genes are present on the X chromosome?

About 900 genes.

Why do all humans need at least one X chromosome?

Many genes on the X chromosome are essential for both males and females.

How many genes are on the Y chromosome?

About 55 genes.

What chromosomes do males typically have?

One X and one Y chromosome.

What chromosomes do females typically have?

Two X chromosomes.

What determines the sex of offspring in humans?

The sperm from the father.

What causes Klinefelter’s syndrome?

Having two X chromosomes and one Y chromosome.

What causes Turner’s syndrome?

Having one X chromosome and no other sex chromosome.

What gene on the Y chromosome determines male development?

SRY, the testis-determining factor.

What do developing testes secrete?

Testosterone.

What hormone causes female reproductive development?

Oestradiol.

Who discovered sex linkage?

Thomas Morgan.

What organism was used to study sex linkage?

The fruit fly, Drosophila.

What is the genotype of a male Drosophila?

XY.

What is the genotype of a female Drosophila?

XX.

What allele is dominant for eye color in Drosophila?

Red eyes.

What allele is recessive for eye color in Drosophila?

White eyes.

What is the difference between gene linkage and sex linkage?

Gene linkage occurs when genes are close together on a chromosome, while sex linkage is when a gene is located on a sex chromosome.

What chromosome are most sex-linked genes located on?

The X chromosome.

Why are males more likely to exhibit sex-linked recessive disorders?

They only have one X chromosome.

What is an example of a sex-linked disorder?

Haemophilia.

What protein is defective in haemophilia?

Factor VIII.

What does Factor VIII do?

It aids in blood clotting.

How is haemophilia treated?

By infusing Factor VIII purified from donor blood.

Why is haemophilia less common in females?

They must inherit two copies of the recessive allele to have the disorder.

What is the estimated frequency of haemophilia in boys?

1 in 10,000.