Genetics: Heredity, Traits and Genes

• The study of genetics helps people to learn about their genetic makeup.
• Over 12 million Americans sought to learn about their genetic makeup as of 2018.

Hippocrates & Genetics

• Hippocrates believed that "pangenes" travel from each body part to the eggs or sperm.
• This turned out to be incorrect, because reproductive cells aren't made of somatic cells particles, and changes in somatic cells don't affect gametes.
• The blending hypothesis was rejected because it couldn't explain why traits that disappear in one generation can reappear in later ones.

Genetics, Heredity, Characters & Traits

• Heredity involves the transmission of traits from one generation to another.
• Genetics is the scientific study of heredity, pioneered by Gregor Mendel.
• A character is a heritable feature that varies among individuals.
• A trait is a variant of a character.
• Mendel crossed pea plants, tracing traits from generation to generation.
• He hypothesized that alternative versions of genes (alleles) determine heritable traits.
• The advantage of using pea plants is that their ability to mate can be strictly controlled.
• Petals enclose the reproductive organs.
• Carpels contain eggs, and stamens release sperm-containing pollen.
• Self-fertilization occurs naturally due to this configuration.
• Mendel ensured self-fertilization by covering pea plants with a bag.
• To cross-fertilize, Mendel removed stamens, transferred pollen, and planted seeds.
• He observed offspring (F₁) traits and P (parental) generation traits.

Mendel's Experimental Success

• Mendel used pea plants with easily observed characters (7) with dominant and recessive traits.
• Flower color can be purple (dominant) or white (recessive).
• Flower position can be axial (dominant) or terminal (recessive).
• Seed color can be yellow (dominant) or green (recessive).
• Seed shape can be round (dominant) or wrinkled (recessive).
• Pod shape can be inflated (dominant) or constricted (recessive).
• Pod color can be green (dominant) or yellow (recessive).
• Stem length can be tall (dominant) or dwarf (recessive).
• True-breeding plants, through generations of self-fertilization, produce the same variety.
• A cross is cross-fertilization of two different varieties, also known as hybridization.
• Hybrids are offspring of two different varieties.
• The P generation is the parental generation of true-breeding parents.
• The F₁ generation (Filial) is the hybrid offspring.
• The F2 generation is the offspring of self-fertilizing F₁ plants or from F₁ plants fertilizing each other.

Mendel's Experiment & Ratios

• Mendel crossed true-breeding purple-flowered plants with true-breeding white-flowered plants.
• The F1 generation (hybrids) resulted in 100% purple flowers.
• Upon fertilization among F₁ plants, the F2 generation appeared with 3:1 ratio.
• 75% of the F2 plants had purple flowers, and 25% had white flowers.
• Heritable factor in the white-flowered plants was not lost, but masked in the F₁ generation when purple flowers were present.
• F1 plants must carry two factors for the flower color character, one for purple and one for white.
• From his experiments, Mendel developed 4 hypotheses.

Mendel's Hypotheses

• Alternative versions of genes (alleles) account for variations in inherited characters.
• An organism inherits two alleles for each character, one from each parent.
• Homozygous organisms have two identical alleles for a gene.
• Heterozygous organisms have two different alleles for a gene.
• The dominant allele determines the organism's appearance, while the recessive allele has no noticeable effect.
• A sperm or egg carries only one allele for each inherited character.
• Allele pairs separate (segregate) during gamete production.
• The segregation law states that allele pairs separate (segregate) from each other during the production of gametes.
• Mendel's hypotheses also explain the 3:1 ratio observed in the F2 generation.

Genotypes & Phenotypes

• True-bred adults include purple-flowered adults (PP) with P gametes, and white-flowered adults (pp) with p gametes.
• The F₁ hybrid generation results in all purple-flowered phenotypes (observable trait).
• A Pp genotype (genetic makeup; alleles) exists.
• The F2 generation results in phenotypic ratios of 3:1.
• Genotypic ratio is 1:2:1 (1PP:2Pp:1pp).
• A Punnett square shows the four possible allele combinations when gametes combine.

Alleles & Chromosomes

• Every diploid cell has pairs of homologous chromosomes.
• Each labeled chromosome band on Fig. 9.5 represents a gene loci.
• A gene loci is a specific location along the chromosome.
• An allele is an alternative version of a gene residing at the same loci on homologous chromosomes.
• Genotype: genetic makeup of gene.
• Chromosomes in a homologous pair carry alleles of the same genes at the same locations.
• Homozygous genotype refers to same allele at same gene loci of homologous chromosome (Genotypes PP and aa).
• Heterozygous genotype has different alleles at the same gene loci of homologous chromosome (Genotypes Bb).

Monohybrid & Dihybrid cross

• A cross between two individuals heterozygous for one character is called a monohybrid cross.
• A dihybrid cross is a cross of two organisms that are each heterozygous for two characters being followed.
• Mendel's law of independent assortment states that a pair of alleles segregate independently of other allele pairs during gamete formation.
• Progressive retinal atrophy (PRA) demonstrates independent assortment of two genes in Labrador retrievers.
• B allele dominant over b allele.
• N allele dominant over n allele, and n allele causes PRA; _ = either dominant or recessive allele.

Testcross

• Offspring of a testcross with a homozygous recessive individual can reveal an unknown genotype.

Segregation & Fertilization

• The rule of multiplication calculates the probability of two independent events both occurring.
• The rule of addition calculates the probability of an event that can occur in alternative ways.

Human Traits & Genetic Inheritance

• Many human traits follow Mendel's laws.
• Family pedigrees can help determine individual genotypes.
• Widow's peak versus straight hairline and then inferred genotyping.
• Dominant trait does not mean that it is normal.
• Dominance means that a heterozygote displays the dominant phenotype.
• Recessive allele is only seen in a homozygote.
• Mutant traits are less common in nature.
• Genetic disorders are inherited as dominant or recessive traits controlled by a single gene.
• Recessive allele disorders can be carried via a heterozygote.
• Dominant allele disorders cannot be "caried" by heterozygotes.
• Medical technologies can provide insights into our genetics.
• Genetic testing is available for potential parents.
• Decisions can be made on family planning due to genetic testing.
• Several technologies are capable of detecting genetic conditions in fetal cells.
• Amniocentesis and chorionic villus sampling allow for karyotyping.
• Blood tests for a variety of tests.
• Fetal imaging and ultrasounds can detect anatomical deformities.
• Newborn screening such as for Phenylketonuria (recessively inherited).

Ethical Genetic Considerations

• Ethical considerations exist in relation to genetic testing/screening.
• Weigh reduced human suffering versus the value of life.

Complete & Incomplete dominance

• Mendel's laws are valid, but genotype often does not dictate phenotype in the simple way Mendel's laws describe.
• Mendel's pea crosses always looked like one of the two parental varieties.
• Dominant allele has the same phenotypic effect whether present in one or two copies.
• Incomplete dominance: F₁ hybrids appearance falls between the two parental varieties phenotypes.
• Phenotype of heterozygote is intermediate between the phenotypes of the two homozygotes

Codominance

• The ABO blood group phenotype in humans is controlled by three alleles, that produce a total of four phenotypes.
• IA and IB alleles are codominant.
• Both alleles are expressed in heterozygous individuals.

Pleiotropy

• Pleiotropy occurs when gene influences multiple characters.
• Sickle-cell disease is a human example of pleiotropy.
• It affects the type of hemoglobin produced, the shape of red blood cells, and causes anemia and organ damage.
• Sickle-cell and nonsickle alleles are codominant, and carriers have shown increased resistance to malaria.

Polygenic Inheritance

• Polygenic inheritance features the additive effects of two or more genes on a single phenotypic character.
• Human height is an example of polygenic inheritance.
• The environment also affect genetic-based traits.

Chromosome Theory

• Chromosome theory of inheritance holds that genes occupy specific loci on chromosomes and chromosomes undergo segregation and independent assortment during meiosis.
• Mendel's laws correlate with chromosome separation in meiosis.
• Linked genes are located close together on the same chromosome and tend to be inherited together.
• Crossing over creates new combinations of alleles in gametes.
• Meiosis in heterozygous (PpLI) sweet-pea plants mostly yields two genotypes of gametes (PL and pl).
• This results in less than equal numbers of the four types of gametes due to linked pollination.

Recombination & Inheritance

• Thomas Hunt Morgan's fruit fly experiment demonstrates the role of crossing over.
• Wild-type is the most common trait in nature.
• Mutant traits are less dominant.
• In fruit flies, for body/wing qualities, offspring either feature parental phenotypes or recombinant phenotypes.
• The percentage of recombinant offspring among the total is the recombination frequency.

Genetic Mapping

• Recombination frequencies in gene mapping can determine the relative positions of genes on chromosomes.
• A genetic map is an ordered list of the genetic loci along a chromosome.
• Alfred H Sturtevant developed a technique assuming the chance of crossing over is equal at all points along the chromosome.
• Farther apart the two gene, the more points between result in crossing over.
• Sturtevant assigned relative position of the fruit fly genes from the crossing data.
• Sturtevant's method of mapping discovered how genes helped established the relative positions of many fruit fly genes.

Chromosomal Sex Determination

• In mammals, XY sex chromosomes denotes males and XX denotes females.
• Y chromosome possesses genes for testes development, whereas lack of it denotes ovaries.
• In addition to the sex chromosomes, humans have 44 autosomes (nonsex chromosomes).
• Non-mammalian species exhibit other chromosomal systems that determine sex.
• In some animals, the sex depends on environmental temperature.

Sex Linked Inheritance

• Genes on either sex chromosome is called a sex-linked gene.
• X chromosome has X-linked genes that control traits unrelated to sex.
• The inheritance of white eye color in the fruit fly is an illustration of an X-linked inherited recessive trait.
• It is carried in the X allele, with no gene on Y.
• Dominant allele in this case codes red eyes, whereas recessive allele codes white eyes

Male & Female Sex Determination

• Red-eye males are XRY while while-eyed males are XY, and male's phenotype only depends of the X chromosome. There are a number of X and crosses that can occur.
• Possible crosses include: -XRXR female x XY Male cross features all red-eye offspring
• Genotypes for this cross show XRXr and XRY -XRXr female x XRY male cross exhibits 3 red and 1 white phenotype.
• Genotypes for this cross show, XRXr, XRY, XrXR, and XY -XRXr female x XY male cross features exhibits 2 red and 2 white phenotype.
• Genotypes for this cross show, XRXr, XRY, XrXR, and XY
• Most X-linked human disorders are due to recessive alleles, and so are seen mostly in males.
• A male receiving a single X-linked recessive allele from his mother will have the disorder.
• A female only receives from both parents to be affected.
• Y chromosomes can provide data about recent evolutionary history because they are passed intact from father to son.
• Approx. 8% of males living in central Asia exhibit Y chromosomes with striking genetic similarity.
• Genetic heritage to a man living about 1,000 years ago.
• Mongolian ruler Genghis Khan may be responsible for the spread of the telltale chromosome to nearly 16 million male descendants.