Chapter 14: Mendelian Inheritance

BIOL 1020 Rebecca Riggs

Mendelian Inheritance

the study of how traits are passed from parents to offspring using Mendel’s genetic principals

What are the four core concepts of Mendelian Inheritance?

1. Foundations for modern transmission genetics

2. The Principle of Segregation

3. The Principle of Independent Assortment

4. Mendel’s laws apply to all sexually reproducing eukaryotes, including humans

Why is Mendel considered the father of genetics?

because his experiments established the fundamental laws explaining how hereditary traits are transmitted

What was Mendel’s experiment goal?

to test whether predictable patterns in offspring occurred when crossing true-breeding plants

What did Mendel hope to achieve if he observed patterns?

he wanted to use those patterns to predict the outcomes of future crosses

What is meant by “true-breeding” in Mendel’s studies?

Plants that consistently produce offspring identical in a specific trait when self-fertilized

What organism did Mendel use for his genetic studies?

the garden pea plant (pisum sativum)

What experimental method did Mendel use with pea plants?

hybridization

What is hybridization?

interbreeding between different varieties to analyze trait inheritance

What seven contrasting traits did Mendel study?

1. Seed Color

2. Seed Shape

3. Pod Color

4. Pod Shape

5. Flower Color

6. Flower Position

7. Plant Height

Why were these traits useful for Mendel’s experiments?

each trait had clear dominant and recessive forms, making inheritance patterns easy to observe

Why could pea plants self-fertilize?

their flowers contain both sperm and egg producing structures

What did Mendel remove to prevent self-fertilization?

The anther, which produces pollen (sperm cells)

Why did Mendel remove the anther?

to ensure that fertilization occurred only with pollen from a selected plant, allowing controlled crosses

How did Mendel transfer pollen between plants?

using a paintbrush to collect pollen from one plant and place it on the stigma of another

How did he prevent unintended fertilization from other pollen?

by covering the fertilized flowers with a cloth bag

Why was covering the flowers necessary?

to keep out airborne or insect-carried pollen that could contaminate the cross

What were the parent traits Mendel crossed in this experiment?

one true-breeding plant with yellow seeds and one with green seeds

What were the results of crossing these two parental plants?

all offspring (F₁ generation) produced yellow seeds

What conclusion did Mendel draw from this result?

the yellow seed trait is dominant over the green seed trait because it appeared in every F₁ offspring

What does “true-breeding” mean in Mendel’s experiments?

true-bleeding plants consistently produce offspring with the same trait when self-fertilized, showing genetic purity for that characteristic

Did the pollen donor’s trait affect F₁ appearance?

no, the pollen donor’s trait did not affect the appearance of the F₁ generation

What concept did Mendel propose to explain this?

Mendel suggested the existence of hereditary factors, which we now call genes

Define allele.

different forms of a gene

Define genotype.

the combination of alleles an organism has

Define phenotype.

the physical expression of the genotype (observable traits)

What is the relationship between genotype and phenotype?

the genotype determines the phenotype, meaning the combination of alleles produces the observable characteristics

What did Mendel do after producing the F₁ generation?

he allowed the F₁ plants to self-fertilize to produce the F₂ generation

What ratio appeared in the F₂ generation for all seven traits?

a consistent 3:1 ratio of dominant to recessive phenotypes

What key observations did Mendel make about the recessive trait?

the recessive trait reappeared in the F₂ generation, proving it was not lost, just hidden in F₁

What did Mendel’s key observations reveal about the nature of genes?

each trait is controlled by two alleles that separate during gamete formation and recombine during fertilization

What does the Principle of Segregation state?

each organism carries two alleles for a trait, but these alleles separate during gamete formation so each gamete receives only one

How does this principle explain the 3:1 ratio?

when gametes combine randomly, dominant and recessive alleles pair in predictable proportions, producing 3 dominant : 1 recessive phenotypes

What are the three possible genotypes for a single gene?

homozygous dominant (AA), heterozygous (Aa), homozygous recessive (aa)

Which genotype combinations express the dominant phenotype?

both AA and Aa individuals show the dominant trait

What is the purpose of a testcross?

to determine whether an individual showing a dominant trait is homozygous dominant or heterozygous

How is testcross performed?

by crossing the dominant-phenotype individual with one that is homozygous recessive

What do the results of a testcross indicate?

• if all offspring show the dominant trait → parent is homozygous dominant

• if half show dominant and half show recessive → parent is heterozygous

How does meiosis support Mendel’s Principle of Segregation?

during meiosis I, homologous chromosomes (carrying different alleles) separate, ensuring each gamete gets only one allele

Why is meiosis essential to Mendelian genetics?

it explains how genetic information is halved in gametes and restored in offspring when gametes fuse during fertilization

What is the difference between maternal and paternal chromosomes in meiosis?

they carry different alleles of the same genes, which are separated into different gametes during meiosis

What is incomplete dominance?

it occurs when the heterozygous phenotype is intermediate between both homozygous phenotypes

How does incomplete dominance differ from complete dominance?

in complete dominance, the dominant allele fully masks the recessive one; in incomplete dominance, neither allele is completely dominant, producing a blended appearance

Give an example of incomplete dominance.

cross true-breeding red and white snapdragons result in pink offspring, showing an intermediate trait

What does incomplete dominance reveal about gene expression>

it shows that alleles can have additive effects, where both contribute partially to the phenotype

What is codominance?

a genetic condition where both alleles are fully and simultaneously expressed in the heterozygote

How is codominance different from incomplete dominance?

in codominance, both traits appear separately and distinctly (not blended), while incomplete dominance produces an intermediate phenotype

Give a classic example of codominance in humans.

AB blood type, where both A and B alleles are expressed equally on the red blood cells

What does codominance demonstrate about allele function?

each allele produces a detectable product — both remain active inn heterozygous individuals

What is the addition rule of probability?

used when two outcomes cannot occur simultaneously; their probabilities are added together

Example of the addition rule in genetics.

the probability of producing either AA or Aa offspring = P(AA)+ P(Aa)

What is the multiplication rule of probability?

used when two independent events can occur together; their probabilities are multiplied

Example of the multiplication rule in genetics.

the probability of offspring being AaBb = P(Aa) x P(Bb)

Why are probability rules important in genetics?

they allow prediction of genotypic and phenotypic ratios without constructing full Punnett squares for complex crosses

What does the Principle of Independent Assortment state?

alleles of different gens assort independently into gametes; the inheritance of one trait does not influence another

How does independent assortment differ from segregation?

segregation deals with separation of alleles of a single gene, while independent assortment involves multiple genes on different chromosomes

What type of plant cross demonstrated independent assortment?

crossing two plants homozygous for two traits (e.g., yellow/green seed color and round/wrinkled seed shape)

What were the F₁ results of the dihybrid cross, crossing two homozygous plants?

all F₁ offspring had yellow, round seeds, showing both dominant traits

Why does independent assortment occur biologically?

because homologous chromosome pairs align randomly during metaphase I of meiosis

What happens when two F₁ heterozygotes (AaBb x AaBb) are crossed?

the F₂ generation shows four phenotype combinations in a 9 : 3 : 3 : 1 ratio

What does the 9 : 3 : 3 : 1 ratio represent?

• 9 = dominant for both traits (A_B_)

• 3 = dominant A, recessive b (A_bb)

• 3 = recessive a, dominant B (aaB_)

• 1 = recessive for both (aabb)

How are these ratios calculated?

using the multiplication rule, multiplying probabilities for each independent trait

What key conclusion did Mendel make from dihybrid crosses?

traits are inherited independently, leading to new combinations in offspring

How do chromosome alignments demonstrate independent assortment?

genes on different chromosomes can line up in two equally likely orientations, creating genetic variation in gametes

What happens in the first type of alignment?

The B allele (dark red) goes to the same pole as the A allele (dark blue), producing AB and ab gametes

What happens in the second type of alignment?

the b allele (light red) travels with the A allele, producing Ab and aB gametes

What is the resulting gamete ratio from each alignment?

a 1:1 ratio of gamete types in both alignments

What is the genetic significance of independent alignment?

it’s a major source of genetic diversity, ensuring unique allele combinations in offspring

What are Mendel’s two fundamental laws of inheritance?

1. Law of Segregation

2. Law of Independent Assortment

What is the Law of Segregation?

each individual has two alleles for every gene, which separate equally into gametes during meiosis

What is the Law of Independent Assortment?

alleles of different genes segregate into gametes independently of one another

How did Mendel derive these laws?

from consistent results in his monohybrid and dihybrid crosses using pea plants

What is the biological basis of the Law of Segregation?

the separation of homologous chromosomes during Meiosis I, ensuring each gamete gets one allele per gene

What is the biological basis of the Law of Independent Assortment?

the random orientation of chromosome pairs during metaphase I, which produces genetically unique gametes

Why are Mendel’s laws considered universal?

because they apply to all sexually reproducing eukaryote, including humans

What is epistasis?

the interaction of two or more genes where one gene’s expression masks or modifies the expression of another gene

How does epistasis affect phenotypic ratios?

it alters expected Mendelian ratios, such as changing the 9:3:3:1 ratio to 13:3 or other variations

Give an example of an epistatic relationship.

interaction between a pigment gene and an inhibitor gene that controls color expression

Why does epistasis occur?

because multiple genes influence the same biochemical pathway, where one gene’s product may be required for another’s effect to appear

How does epistasis differ from dominance?

dominance occurs between alleles of the same gene, while epistasis occurs between different genes

What pigment gives pea flowers their purple color?

anthocyanin, a purple pigment

How is the anthocyanin produced in pea plants?

through a two-step biochemical process controlled by the C and P genes

What must be true for a pea plant to have purple flowers?

both C and P genes must produce functional proteins (both dominant alleles present)

What happens if either gene (C or P) is nonfunctional?

anthocyanin cannot be produced, and the flower will be white

What genotype combination did Mendel use in this example?

he crossed two plants with the genotype CcPp to observe pigment inheritance

What are pedigrees used for in human genetics?

to trace inheritance patterns and determine genotypes across multiple generations

What do dominant traits look like in pedigrees?

they appear in every generation, with at least one affected parent

What do recessive traits look like in pedigrees?

they may skip generations, appearing only when both parents carry the recessive allele

Why are pedigrees important for studying human inheritance?

because controlled crosses are not possible in humans, so family histories reveal inheritance patterns instead

In a dominant inheritance pattern, are males and females equally affected?

yes, sex does not influence expression of autosomal dominant traits