1. Ch. 11 Introduction to Genetics

2. Experiments of Gregor Mendel
11.1, Work of Gregor Mendel
Experiments of Gregor Mendel
Individual’s looks determined by traits passed to offspring from parents
heredity: delivery of characteristics from parent to offspring
genetics: scientific study of heredity

3. 11.1, work of Mendel Mendel worked w/ ordinary garden peas
peas = “model system”
Model system because they
have few traits and they
grow fast!
Mendel did experiments in
short time that would take
years to do. w/ humans

4. 11.1, work of Mendel Role of Fertilization
male part of each flower makes pollen, which contains sperm (male reproductive cells)
female part of each flower makes eggs (female reproductive cells)
pea flowers normally “self-pollinating” (sperm fertilize eggs from w/in same flower)

5. 11.1, work of Mendel
fertilization: in sexual reproduction, egg & sperm join to produce new cell
in peas, new cell develops into tiny embryo (the peas in the pod).
Because peas self-pollinate, they inherit all its characteristics from its single parent plant

6. 11.1, work of Mendel
Mendel had peas that were “true-breeding” (produced offspring w/ identical traits to themselves)
trait: specific characteristic of individual (seed color, plant height, etc.) that may vary from 1 individual to another
Mendel decided to “cross-pollinate” his stocks (transfer pollen to cause 1 plant to reproduce w/ another plant)

7. 11.1, work of Mendel
cross-pollination let Mendel breed plants w/ traits different from their parents & study results
hybrids: offspring of crosses between parents w/ different traits
Genes & Alleles
original organisms studied = P (parental) generation
offspring of P = F1 generation

8. 11.1, work of Mendel
For all 7 traits Mendel studied, every offspring looked identical to the trait of 1 of their parents (no “blended” traits)
Other parent’s trait “disappeared”

9. 11.1, work of Mendel
Mendel’s 1st conclusion: an individual’s characteristics are determined by factors passed from 1 generation to next
factors = genes
alleles: genes for different forms of a given trait
In the picture, the
Alleles are “R” and “r”
F1 recieves 1 allele from
each parent.

10. 11.1, work of Mendel Mendel’s 2nd conclusion: principle of dominance
some alleles are dominant & others are recessive
dominant allele= organism will show that form of trait (represented with : CAPITAL LETTER)
recessive allele= organism will exhibit that form only if no dominant allele is present (represented with: lowercase letter)

11. 11.1, work of Mendel Segregation
Each parent has 2 alleles, the genes segregate from each other, so each sex cell only carries one allele.
WHERE DID THE RECESSIVE GO?!?
Solution: Mendel let his F1 hybrids self-pollinate, making F2 hybrids

12. Results of crossing F1 Generation
11.1, work of Mendel
Results of crossing F1 Generation
recessive alleles reappeared in F2 THEY CAME BACK! (1/4 had recessive trait)
Dominant allele hid recessive allele in F1
Reappearance of recessive trait indicated that, at some point, short allele separated from tall allele.

13. 11.1, work of Mendel Formation of Gametes
Assume each F1 plant has inherited 1 tall allele from its tall parent & 1 short allele from its short parent.
each F1 plant in Mendel’s cross produced 2 kinds of gametes — ½ w/ tall allele (T) & ½ w/ short allele (t)
This could go two ways….
1. If each 2 gametes with “t” allele paired to produce F2 plant, that plant was short (tt)
2. If either of the two gametes in F2 plant was T, that plant was tall (TT or Tt).

15. 11.2, applying Mendel’s principles
Probability & Punnett squares
Punnett squares use mathematical probability to help predict genotypes & phenotypes in genetic crosses
probability: likelihood that particular event will occur
past outcomes DO NOT affect future ones

16. 11.2, applying
To find probability of multiple events, multiply probabilities of each
example: probability of flipping head is 1/2, so probability of 3 heads in a row is: 1/2 × 1/2 × 1/2 = 1/8
How the alleles segregate during gamete formation is just as random as coin flip

17. 11.2, applying
How alleles segregate during gamete formation is just as random as coin flip
ex: F1 parent w Tt genotype has 1/2 chance of t in gamete, so chance of F2 w/ tt is 1/2 × 1/2 = 1/4, & Mendel had 1/4 short

18. 11.2, applying
homozygous: w/ 2 identical alleles for trait (ex.: TT or tt)
Homozygous dominant: TT (2 dominant)
Homozygous recessive: tt (2 recessive)
heterozygous: w/ 2 different alleles for trait (ex.: Tt)

19. 11.2, applying Genotype & Phenotype genotype: genetic makeup for trait
phenotype: physical trait (what you see)
genotype determines phenotype, but organisms with the same phenotype may have a different genotype
ex.: TT or Tt both look tall

20. Punnett square for 1-factor cross
11.2, applying
Punnett square for 1-factor cross
write genotypes of parents & possible alleles from each
write one parent’s alleles across top & other down side
fill boxes
count ratios, from most to least dom.
Genotype ratio = 1:2:1
Phenotype ratio = 3:1

21. 11.2, applying
principle of independent assortment: genes for different traits can segregate independently during formation of gametes
monohybrid cross: genes for 1 trait
dihybrid cross: 2 different traits
trihybrid cross: 3 traits

22. 11.2, applying
Mendel used true-breeding parent plants to create F1 that were heterozygous for traits (dihybrid)
crossing F produces 4 combo phenotypes (G & Y, G & y, g & Y, or g & y)

23. 11.2, applying, cont
since seed color & pod color didn’t affect each other, Mendel concluded that 1 trait had no effect on another during gamete formation (independent assortment)

24. 11.3, other patterns of inheritance
beyond dominant & recessive alleles
some alleles are neither dominant nor recessive ( called incomplete dominance or codominance)

25. 11.3 Other Patterns
Many genes exist in several different forms (have multiple alleles)
Many traits are produced by the interaction of several genes (polygenic traits)

26. 11.3, other patterns beyond dominant & recessive
incomplete dominance: 1 allele is not completely dominant over another; phenotype is intermediate (ex: red & white alleles making pink flowers)
codominance: both alleles show up full strength (A & B blood)
multiple alleles: more
than 2 possible genes
(A, B, or O blood)

28. Let’s try it out.. (Type A) (Type AB) IA i IA IAIA IAi (Type A) IB
IAIB
(Type AB)
Ibi
(Type B)

29. 11.3, other patterns genes & environment
environmental conditions can affect gene expression & influence genetic traits
temperature, nutrition, diseases during development. Remember Jurassic Park??

30. 11.4, meiosis
chromosome #
diploid cells of most adult organisms have 2 sets of inherited chromosomes w/ 2 complete sets of genes
chromosomes: strands of DNA & protein inside cell nucleus
genes located in specific positions on chromosomes

31. 11.4, meiosis, cont chromosome #, cont
diploid cell: any cell w/ 2 sets of chromosomes, 1 from each parent
homologous chromosomes: same type; have genes for same traits
haploid cell: cell w/ 1 set of chromosomes, generally gametes

32. 11.4, meiosis, cont phases of meiosis
2 divisions, each w/ stages similar to mitosis
meiosis I (a.k.a. reduction division) separates homologous chromosomes into 2 haploid cells, still w/ 2 chromatids for each chromosome
meiosis II (a.k.a. mitotic division) splits sister chromatids in each cell from meiosis I, forming 4 haploid gametes

33. 11.4, meiosis, cont phases, cont meiosis I prophase I
like mitosis, chromosomes, spindle fibers, centrioles form, nuclear envelope breaks down
homologous chromosomes pair up, forming tetrads (4 similar chromatids)

34. 11.4, meiosis, cont phases, cont meiosis I, cont prophase I, cont
crossing-over: homologous chromosomes “swap” genes
chromatids overlap sections
crossed sections cut & spliced, exchanging genes
produces new combinations of alleles

35. 11.4, meiosis, cont phases, cont meiosis I, cont
metaphase I: tetrads line up across cell’s center w/ spindle attached to chromosomes
anaphase I: spindle fibers pull each homologous chromosome pair toward opposite ends of cell
telophase I & cytokinesis: nuclear membranes form & cytoplasm splits

36. 11.4, meiosis, cont phases, cont meiosis II
prophase II: chromosomes & spindles form
metaphase II: chromosomes line up & spindles attach to chromatids
anaphase II: chromatids separate
telophase II & cytokinesis: nuclear membranes form & cytoplasm splits

37. 11.4, meiosis, cont phases, cont gametes to zygotes
haploid cells produced by meiosis II are gametes
in males, these gametes are called sperm (plant sperm enclosed in pollen)
in female animals, usu. only 1 cell from meiosis becomes egg (other 3 form small cells called polar bodies)
in female plants, might make 4 eggs or 1 egg & 3 polar bodies

38. 11.4, meiosis, cont phases, cont gametes to zygotes, cont
fertilization: fusion of male & female gametes
zygote: cell formed by fertilization; forms new organism w/ new combination of genes diff. from parents
gene linkage & gene maps
alleles of diff. genes tend to be inherited together when those genes are on same chromosome

39. 11.4, meiosis, cont phases, cont gene linkage & maps, cont
Thomas Hunt Morgan studied genetic traits & chromosomes in fruit flies, w/ 2 conclusions:
each chromosome is actually a group of linked genes
chromosomes assort independently, not individual genes
gene map: Alfred Sturtevant figured out that crossing-over was more frequent when genes were closer together on chromosome