Patterns of Inheritance and Genetics

Introduction to Genetics and Heredity

• Genetics: Defined as the scientific study of heredity and hereditary variation.

• Heredity: The process involving the transmission of traits from one generation to the next.

• Gregor Mendel: Known as the "Father of Modern Genetics." He conducted systematic crossing experiments using different varieties of garden peas to understand the patterns of inheritance.

Methodology of Mendel’s Crossing Experiments

• Crossing (Hybridization) in Plants: Mendel utilized a specific technique to control plant reproduction:

  • Removal of Stamens: He removed the stamens (the male reproductive organs that release sperm-producing pollen) from a purple flower to prevent self-pollination.

  • Pollen Transfer: He transferred pollen from the stamens of a white flower to the carpel (the female reproductive organ that produces eggs) of the purple flower.

  • Maturation: The pollinated carpel matured into a seed pod.

  • Offspring Development: Mendel planted the seeds from the pod to observe the resulting offspring, known as the $F_1$ generation.

• Anatomy of the Flower Used:

  • Petal: The colorful attracting structure.

  • Stamen: Releases sperm-producing pollen.

  • Carpel: Produces eggs.

The Choice of Garden Peas ($Pisum sativum$)

Mendel selected the garden pea for several practical and biological reasons:

• Short Generation Time: Allowed for the study of many generations in a relatively short period.

• Variety: They were available in many distinct varieties.

• Self-Pollination: Naturally self-pollinating, making it easier to maintain pure lines.

• Controlled Crossing: It was easy to manually control which plants mated with which.

• True Breeding Varieties: Mendel could start with plants that, when self-pollinated, always produced offspring with the same traits.

Mendel’s Seven Pea Plant Traits

Table 13.1 details the specific traits Mendel studied, identifying the dominant and recessive forms for each:

• Seed Shape: Dominant = Round; Recessive = Wrinkled

• Seed Color: Dominant = Yellow; Recessive = Green

• Pod Texture: Dominant = Smooth; Recessive = Wrinkled

• Pod Color: Dominant = Green; Recessive = Yellow

• Flower Color: Dominant = Purple; Recessive = White

• Flower Position: Dominant = Along Stem; Recessive = At Tip

• Stem Length: Dominant = Tall; Recessive = Short

Inheritance in Modern Terminology

• Genes and Loci: Members of the same species share traits because their chromosomes carry the same genes. The DNA sequence of each gene occurs at a specific location on a chromosome called a locus.

• Alleles: Different versions of the same gene.

• Homozygous: An individual carrying two identical alleles for a specific gene (e.g., $AA$ or $aa$).

• Heterozygous: An individual carrying two different alleles for a specific gene (e.g., $Aa$).

• Genotype: The particular set of alleles that an individual carries (the genetic makeup).

• Phenotype: The observable physical or physiological traits of an individual (e.g., flower color).

• Dominant vs. Recessive: An allele is dominant ($A$) when its effect masks that of a recessive ($a$) allele when paired together.

• Hybrids: These are heterozygous offspring resulting from a cross between two different homozygous parents.

Designating Generations

• $P$ Generation: The parental generation (the original true-breeding parents).

• $F_1$ Generation: The first filial generation; the offspring resulting from crossing two parents with contrasting traits.

• $F_2$ Generation: The second filial generation; resulting from either self-pollination of $F_1$ individuals or crossing two $F_1$ individuals.

Punnett Squares and Meiosis

• Punnett Square: A grid system used to predict the genotypic and phenotypic outcomes of a genetic cross.

• Gamete Distribution: The distribution of alleles into gametes follows the principles of meiosis, ensuring each gamete receives only one allele from each gene pair.

Mendel's First Law: The Law of Segregation

• Definition: During the formation of gametes, the paired unit factors (alleles) segregate or separate randomly so that each gamete receives one or the other with equal likelihood.

• The Monohybrid Cross: A cross where parents are heterozygous for the same single characteristic.

  • Initial Cross: Between two true-breeding varieties with contrasting traits (e.g., Purple $AA$ $\times$ White $aa$).

  • $F_1$ Results: 100% of the offspring show the dominant trait (e.g., Purple). The white trait is masked.

  • $F_2$ Results: When $F_1$ plants self-pollinated, the white trait reappeared. Mendel consistently observed a $3:1$ phenotypic ratio (3 dominant to 1 recessive).

• Biological Basis: Segregation occurs during Meiosis I when homologous chromosomes separate. The paired condition is restored after fertilization.

Mendel's Second Law: The Law of Independent Assortment

• Definition: During gamete formation, segregating pairs of unit factors (alleles) on homologous chromosomes assort independently of other gene pairs.

• The Dihybrid Cross: A cross where parents are heterozygous for two different traits.

  • Hypothesis of Dependent Assortment: Predicted that traits would stay together (e.g., Round always with Yellow), resulting in only parental combinations. This was not supported.

  • Hypothesis of Independent Assortment: Predicted four different seed phenotypes in the $F_2$. This was supported by Mendel’s actual results.

• Dihybrid Ratios: Mendel consistently observed a $9:3:3:1$ phenotypic ratio among $F_2$ offspring:

  • $9/16$ Round-yellow (Both dominant)

  • $3/16$ Round-green (One dominant, one recessive)

  • $3/16$ Wrinkled-yellow (One recessive, one dominant)

  • $1/16$ Wrinkled-green (Both recessive)

• Biological Basis: This is due to the independent assortment of chromosomes at Metaphase I of meiosis.

The Testcross

• Purpose: To determine the unknown genotype of an individual showing a dominant phenotype.

• Procedure: Cross the unknown individual (could be homozygous dominant or heterozygous) with an individual that is homozygous recessive ($aa$).

• Interpretation:

  • If any offspring show the recessive phenotype, the unknown parent must be heterozygous.

  • If all offspring show the dominant phenotype, the unknown parent is likely homozygous dominant.

Non-Mendelian Inheritance Patterns

Not all genes follow the simple dominant/recessive patterns discovered by Mendel. Notable exceptions include:

• Codominance: Both alleles are expressed equally in the heterozygote.

  • Example: ABO Blood Types. Individuals with genotype $I^A I^B$ have both A and B carbohydrates on their red blood cells (Phenotype AB).

• Incomplete Dominance: The phenotype of the heterozygote is intermediate between the two homozygous phenotypes.

  • Example: Snapdragons. Crossing a red-flowered plant ($RR$) with a white-flowered plant ($rr$) produces 100% pink-flowered offspring ($Rr$). Crossing two pink flowers ($Rr \times Rr$) results in a $1:2:1$ phenotypic ratio (1 Red : 2 Pink : 1 White).

• Polygenic Inheritance: The additive effect of two or more genes on a single phenotypic character. This often leads to continuous variation.

  • Examples: Human skin color, height, and eye color.

  • Continuous Variation: When measured values (like height in inches) are graphed, they produce a bell-shaped curve.

ABO Blood Groups in Detail

• Genotypes and Phenotypes:

  • Phenotype A: Genotypes $I^A I^A$ or $I^A i$. Contains Anti-B antibodies.

  • Phenotype B: Genotypes $I^B I^B$ or $I^B i$. Contains Anti-A antibodies.

  • Phenotype AB: Genotype $I^A I^B$. Contains no antibodies (Universal recipient for cells).

  • Phenotype O: Genotype $ii$. Contains both Anti-A and Anti-B antibodies.

The Chromosomal Basis of Sex Determination

In humans, sex is determined by the sex chromosomes:

• Females: Chromosomal makeup is $44$ autosomes + $XX$. All eggs carry an $X$ chromosome ($22 + X$).

• Males: Chromosomal makeup is $44$ autosomes + $XY$. Sperm carry either an $X$ ($22 + X$) or a $Y$ ($22 + Y$) chromosome.

• Offspring Probability: Sex determination is a $50/50$ chance for each pregnancy.

Sex-Linked (X-Linked) Inheritance

• Definition: Genes located on the sex chromosomes are sex-linked. Most are found on the $X$ chromosome.

• Common X-Linked Recessive Traits:

  • Red-green colorblindness.

  • Hemophilia.

  • Duchenne muscular dystrophy.

• Inheritance Patterns for Colorblindness:

  • Normal female ($X^N X^N$) x Colorblind male ($X^n Y$): All daughters are carriers ($X^N X^n$), all sons are normal ($X^N Y$).

  • Carrier female ($X^N X^n$) x Normal male ($X^N Y$): 50% chance for sons to be colorblind; 50% chance for daughters to be carriers.

  • Carrier female ($X^N X^n$) x Colorblind male ($X^n Y$): 50% chance for any child to be colorblind.

• Hemophilia Case Study: Queen Victoria was a famous carrier of hemophilia, passing the allele to various royal families in Europe ($e.g.$, Alice, Alexandra, and Prince Leopold). Leopold and Alexis of Russia (grandson) were affected.

Human Genetic Disorders and Pedigrees

• Pedigree: A family tree used to track the inheritance of a specific trait (e.g., freckles vs. no freckles).

• Table 9.1: Autosomal Disorders:

  • Recessive Disorders:

    • Albinism: Lack of pigment in skin, hair, and eyes.

    • Cystic Fibrosis: Excess mucus in lungs and digestive tract; potential early death.

    • Phenylketonuria (PKU): Accumulation of phenylalanine; requires special diet (e.g., avoiding Aspartame which contains phenylalanine); can cause developmental disabilities.

    • Sickle-cell disease: Misshapen red blood cells and tissue damage.

    • Tay-Sachs disease: Lipid accumulation in brain cells; childhood death.

  • Dominant Disorders:

    • Achondroplasia: A form of dwarfism.

    • Alzheimer's disease: One specific type involving mental deterioration late in life.

    • Huntington's disease: Middle-age onset of uncontrollable movements and cognitive impairment.

    • Hypercholesterolemia: Excess cholesterol in the blood; heart disease.

Review Questions

• What are the two laws of genetics? The Law of Segregation and the Law of Independent Assortment.

• What kind of crosses did Mendel do to come up with these laws? Monohybrid crosses for the Law of Segregation and Dihybrid crosses for the Law of Independent Assortment.

• X-linked traits are more common in? Males (because they only have one $X$ chromosome).

• If Joe has colorblindness, where did he get the allele? From his mother (males inherit their $X$ chromosome from their mother).

• What are the three alleles for ABO blood types? $I^A$, $I^B$, and $i$.

• Difference between Codominance and Incomplete Dominance: In codominance, both traits appear distinctly in the phenotype (e.g., AB blood). In incomplete dominance, the traits blend to create an intermediate phenotype (e.g., pink flowers from red and white parents).