Mendel, Genes, and Inheritance

Only available on StudyMode
  • Download(s) : 41
  • Published : May 31, 2013
Open Document
Text Preview
Mendel, Genes, and Inheritance
Chapter 12

Why It Matters
 Red blood cells in sickle-cell disease

One amino acid in the wrong position causes the disease

12.1 The Beginnings of Genetics: Mendel’s Garden Peas
 Mendel chose true-breeding garden peas for his experiments  Mendel first worked with single-character crosses  Mendel’s single-character crosses led him to propose the principle of segregation  Mendel could predict both classes and proportions of offspring from his hypotheses

12.1 (cont.)
 Mendel used a testcross to check the validity of his hypotheses  Mendel tested the independence of different genes in crosses  Mendel’s research founded the field of genetics  Sutton’s chromosome theory of inheritance related Mendel’s genes to chromosomes

Blending Theory of Inheritance
 Popular belief until about 1900
• Hereditary traits blend evenly in offspring through mixing of parents’ blood

 Does not explain some observations:
• Extremes do not gradually disappear • Offspring sometimes have different traits than either parent

Gregor Mendel
 Founder/Father of genetics  Austrian monk (18221884)  First to use scientific method to study inheritance

Pea Experiments
 Garden pea (Pisum sativum)
• Easy to grow • Clearly defined characters or traits • True-breeding varieties • Self-fertilized plants (same trait each generation)

• Easy to cross
• Cross-pollination between parents

Garden Pea

Animation: Crossing garden pea plants

Pea Characters

Single-Character Crosses
 P generation (Parents)
• Each pea produced contains an embryo

 F1 generation (Filial)
• First generation

 F2 generation
• Second generation

Flower Color Cross
 P generation
• Purple flowers crossed with white flowers

 F1 generation
• All F1 seeds formed purple flowers • Purple flower offspring crossed

 F2 generation
• Purple flowers (75%) • White flowers reappeared (25%)

Mendel’s First Hypothesis

 Genes for genetic characters occur in pairs
• One gene inherited from each parent • Alleles are different versions of a gene

 Diploid: two copies of each gene

Mendel’s Second Hypothesis
 If two alleles of a gene are different, one allele is dominant over the other • Dominant allele is expressed • Recessive allele is masked

 Recessive alleles only expressed when two copies of the allele present

Mendel’s Third Hypothesis
 Two alleles of a gene segregate (separate) and enter gametes singly • Half the gametes carry one allele, half carry the other allele (haploid) • Principle of Segregation

 Two gametes fuse to produce a zygote that contains two alleles (diploid)

Monohybrid Cross

Terminology
 Homozygous
• Both alleles the same • PP (dominant) • pp (recessive)

 Heterozygous
• Two different alleles • Pp

Terminology
 Genotype
• Genetic constitution of an organism • PP, Pp, pp

 Phenotype
• Outward appearance • Purple flowers, white flowers

Product Rule in Probability
 Probability of two independent events occurring in succession • Individual probabilities multiplied

 Coin flip probabilities
• • • • Heads = ½ Tails = ½ Two heads = ½ × ½ = ¼ Two tails = ½ × ½ = ¼

Rules of Probability

Sum Rule in Probability
 Probability of two different events producing the same outcome • Individual probabilities added

 Probability of a heads and a tails in two tosses:
• First possibility: heads then tails
• Heads = ½, Tails = ½ (½ × ½ = ¼)

• Second possibility: tails then heads
• Tails = ½, Heads = ½ (½ × ½ = ¼)

• Total probability: ¼ + ¼ = ½

Probability in Mendel’s Crosses (1)
 Purple-flowered × white-flowered (PP × pp)
• Probability of PP zygote = ½ × ½ = ¼ • Probability of pp zygote = ½ × ½ = ¼

Probability in Mendel’s Crosses (2)
 Purple-flowered × white-flowered (PP × pp)
• Probability of Pp zygote = ½ × ½ = ¼ • Probability of pP zygote = ½ × ½ = ¼ • Total probability of heterozygote = ¼ + ¼ =...
tracking img