Chapter 12 - Mendel and the Gene Idea

Genetics

Study of heredity, process by which traits are passed from parent to offspring

Blending hypothesis

Pre-1800s idea: traits mix like paint

Particulate hypothesis

Alternative model: traits inherited as discrete units (genes), like a deck of cards

Gregor Mendel

Father of genetics, pea plant experiments (~1857)

Character

Heritable feature that varies among individuals

Trait

Variant of a character

True-breeding

Plants that produce offspring identical to parent when self-pollinated

Hybridization

Crossing of two true-breeding varieties

P generation

Parent individuals in genetic studies

F1 generation

First filial generation, hybrid offspring

F2 generation

Offspring from interbreeding F1 hybrids

Law of segregation

Two alleles for a character separate during gamete formation

Law of independent assortment

Alleles of different genes segregate independently during gamete formation

Mendel’s model principle 1

Alternative versions of genes account for variations in characters

Mendel’s model principle 2

Organism inherits two alleles per character, one from each parent

Mendel’s model principle 3

If alleles differ, dominant allele determines phenotype, recessive masked

Mendel’s model principle 4

Alleles segregate into different gametes during formation

Punnett square

Diagram predicting results of genetic crosses

Homozygous

Two identical alleles for a gene

Heterozygous

Two different alleles for a gene

Genotype

Genetic constitution of an individual

Phenotype

Observable traits, expression of genotype

Testcross

Cross unknown genotype with homozygous recessive to determine genotype

Monohybrid cross

Single gene studied, 3:1 phenotype ratio

Dihybrid cross

Two genes studied, 9:3:3:1 phenotype ratio

Probability

Likelihood an event will occur

Rule of multiplication

Probability of compound event = product of independent probabilities

Rule of addition

Probability of event occurring in multiple ways = sum of probabilities

Complete dominance

One allele fully masks the other

Incomplete dominance

Neither allele completely dominant, heterozygote shows intermediate phenotype (pink flowers)

Codominance

Both alleles expressed equally (e.g., red + white hairs in cattle)

Multiple alleles

More than two possible alleles for a gene (e.g., blood groups)

Pleiotropy

One gene affects multiple traits

Epistasis

One gene affects expression of another gene

Polygenic inheritance

Many genes influence one trait (e.g., height, skin color)

Environmental impact

Phenotype influenced by environment (e.g., sun exposure, nutrition)

Pedigree analysis

Study of family inheritance patterns in humans

Recessive disorders

Genetic disorders inherited as recessive traits (e.g., albinism, cystic fibrosis)

Dominant disorders

Disorders caused by dominant alleles (e.g., achondroplasia, Huntington’s disease)

Achondroplasia

Dominant allele causes dwarfism, ~1 in 25,000 people

Huntington’s disease

Lethal dominant allele, symptoms appear at 35–45 years

Multifactorial disorders

Diseases influenced by genetic + environmental factors (e.g., heart disease, cancer)

Genetic testing

Identifies carriers of genetic diseases (e.g., Tay Sachs, sickle cell, cystic fibrosis)

Fetal testing

Amniocentesis vs chorionic villus sampling (CVS)

Newborn screening

Detects disorders at birth (e.g., PKU)

Who is considered the “father” of genetics? 

Gregor Mendel

Why did Mendel continue some of his experiments to the F2 generation? 

To observe whether or not a recessive trait would reappear  

What was the most significant conclusion that Gregor Mendel drew from his experiments with pea plants? 

Traits are inherited in discrete units and are not the results of the “blending” of traits 

Which of the following statements correctly describes one difference between the law of independent assortment and the law of segregation? 

The law of independent assortment explains the segregation of two or more genes relative to one another  

Which of the following statements correctly describes a monohybrid cross? 

A monohybrid cross involves organisms that are heterozygous for one character 

Which of the following statements correctly describes a dihybrid cross? 

A dihybrid cross involves organisms that are heterozygous for two characters 

In certain plants, the tall trait is dominant to the short trait. If a heterozygous plant is cross with a homozygous tall plant, what is the probability that the offspring will be short  

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