Mendelian Genetics Gregor Mendel – 1822-1884

Asexual Reproduction Bacteria can reproduce as often as every 12 minutes – and may go through 120 generations in one day Thus capable of producing 6 x 1035 offspring per day Bacteria often produce 1 mutation per 1000 replications of DNA So for fast-growing species, mutation is a good way to respond to a changing environment

Why Sex? John Maynard Smith

Sexual reproduction leads to genetic variation via: Independent assortment during meiosis Crossing over during meiosis Random mixing of gametes (sperm and egg)

Independent Assortment

Nonsister chromatids held together during synapsis Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.11 The results of crossing over during meiosis. Anaphase II Daughter cells Recombinant chromosomes

The random nature of fertilization adds to the genetic variation arising from meiosis. Any sperm can fuse with any egg. A zygote produced by a mating of a woman and man has a unique genetic identity. An ovum is one of approximately 8,388,608 possible chromosome combinations (223). The successful sperm represents one of 8,388,608 different possibilities (223). The resulting zygote is composed of 1 in 70 trillion (223 x 223) possible combinations of chromosomes. Crossing over adds even more variation to this.

Mendelian Genetics Gregor Mendel – 1822-1884

Two possible types of inheritance One possible explanation of heredity is a “blending” hypothesis The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea Parents pass on discrete heritable units, later known as genes

Mendel’s time Today Mendel’s garden at Brunn (Brno) Monastery

Some genetic vocabulary Character: a heritable feature, such as flower color Trait: a variant of a character, such as purple or white flowers Garden Pea

Flower Structure

Parental generation (P) TECHNIQUE 1 2 Parental generation (P) Stamens 3 Carpel 4 Figure 14.2 RESEARCH METHOD: Crossing Pea Plants RESULTS 5 First filial generation offspring (F1)

In Mendel’s Experiments: Mendel chose to track Only those characters that varied in an “either-or” manner Mendel also made sure that He started his experiments with varieties that were “true-breeding” In a typical breeding experiment Mendel mated two contrasting, true-breeding varieties, a process called hybridization

Breeding Terminology The true-breeding parents Are called the P (parental) generation The hybrid offspring of the P generation Are called the F1 (filial) generation When F1 individuals self-pollinate The F2 generation is produced

(true-breeding parents) EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers Figure 14.3 INQUIRY: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation?

(true-breeding parents) F1 Generation (hybrids) EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 14.3 INQUIRY: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation?

(true-breeding parents) Purple flowers White flowers EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 14.3 INQUIRY: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? F2 Generation 705 purple- flowered plants 224 white flowered plants

Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

Mendel developed a hypothesis to explain his results that consisted of four ideas Alternative versions of genes (different alleles) account for variations in inherited characters For each character, an organism inherits two alleles, one from each parent If two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance. The other, recessive allele has no effect on a hybrid organism’s appearance The two alleles for each character segregate (separate) during gamete formation

Law of Segregation P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p Figure 14.5 Mendel’s law of segregation.

Law of Segregation P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Genetic makeup: Pp Gametes: 1/2 P 1/2 p Figure 14.5 Mendel’s law of segregation.

Law of Segregation P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Genetic makeup: Pp Gametes: 1/2 P 1/2 p Sperm from F1 (Pp) plant F2 Generation P p Figure 14.5 Mendel’s law of segregation. P Eggs from F1 (Pp) plant PP Pp p Pp pp 3 : 1

PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Phenotype Genotype Purple PP (homozygous) 1 3 Pp (heterozygous) Purple 2 Pp (heterozygous) Purple Figure 14.6 Phenotype versus genotype. pp (homozygous) 1 White 1 Ratio 3:1 Ratio 1:2:1

Test cross TECHNIQUE Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If purple-flowered parent is PP or If purple-flowered parent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs Figure 14.7 RESEARCH METHOD: The Testcross P p Pp Pp pp pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white

Hypothesis of dependent assortment EXPERIMENT YYRR P Generation yyrr Gametes YR yr F1 Generation YyRr Predictions Hypothesis of dependent assortment Hypothesis of independent assortment Sperm Predicted offspring of F2 generation or 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr 1/2 Eggs Figure 14.8 INQUIRY: Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1

Segregation of alleles into eggs Segregation of alleles into sperm Rr  Rr Segregation of alleles into eggs Segregation of alleles into sperm Sperm 1/2 R 1/2 r R R 1/2 R R r Figure 14.9 Segregation of alleles and fertilization as chance events. 1/4 1/4 Eggs r r R r 1/2 r 1/4 1/4

Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16 1/8 Figure 14.UN01

Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16 1/8 Figure 14.UN01 Probability of yyrr = ? A. 1/8 B. 1/16 C. 1/32

Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16 1/8 Figure 14.UN01 Probability of YYrr = ? A. ¼ B. 1/8 C. 1/16

Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16 1/8 Figure 14.UN01 Probability of YxRr = ? (x can be Y or y) A. ½ B. 3/4 C. 3/8 D. 1/16

1/4 (probability of pp)  1/2 (yy)  1/2 (Rr)  1/16 ppYyrr 1/4  1/2  1/2  1/16 Ppyyrr 1/2  1/2  1/2  2/16 PPyyrr 1/4  1/2  1/2  1/16 ppyyrr 1/4  1/2  1/2  1/16 Chance of at least two recessive traits Figure 14.UN02  6/16 or 3/8