BIOL 111 Chapter 14

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Gregor Mendel

(1822-84)

Monk who formed mendelian heredity theory by working with pea plants.

  1. true breeding [vocab 1]
  2. studied one trait at a time
  3. studied mathematics before entering monastery

Technique

Two P [vocab 2] pea plants

  1. Removed stamen from a pea plant flower to prevent cross-pollination
  2. Fertilize Purple plant with pollen from white plant
  3. observe/tally resulting F1 [vocab 3] generation

Results

P (purple × white) → F1 (all purple) → F2 (3 purple : 1 white)

Hypothesis: we inherit two alleles [vocab 4], one from each parent. There are many alleles (a lot more than two for each trait), but we can only have two.

Sperm cells and egg cells only have one of each allele because they carry only one chromosome (one chromosome has several alleles for different traits, not the same trait)

Monohybrid Cross

Follow only one trait through the generations

Punnett Squares

Used to predict possible offspring allele combinations from parents with known phenotypes [vocab 5] and genotypes [vocab 6]

DOMINANT traits assigned capital letter, recessive traits assigned lowercase letter.

  P P
p Pp Pp
p Pp Pp

Homozygous [vocab 7] dominant × homozygous recessive produces four heterozygous [vocab 8] offspring.

Note that a homozygous individual can produce only dominant or only recessive gametes.

Test Cross

In order to determine the genotype of an individual, we cross it with a known homozygous recessive genotype.


Dihybrid Cross

Now following two traits instead of just one.

Example

Suppose YYRR produces yellow, round peas and yyrr produces green, wrinkled peas. Will the traits be inherited together (dependent), or are they inherited separately (independently). (By the way, These traits are indeed inherited independently)

  YR yr
YR YYRR YyRr
yr YyRr yyrr

Produces 3:1 phenotypic ratio


OR

Segregated alleles:

  YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr

Produces 9:3:3:1 phenotypic ratio

Back to Meiosis

Important:

Law of segregation: [vocab 9] each gamete is only going to have one allele (one chromosome per gamete)

Law of independent assortment: [vocab 10] ONLY FOR DIHYBRIDS. Two traits we're following are not on the same chromosome. (independent inheritance)

Probability

Probabilities of scale range from zero to one, and all of the probabilities add to 1 (percentages)

Shortcut: When working with dihybrid Punnett squares, determine probability for individual traits, then multiply them together.

Example

Trihybrid cross:

PpYyRr × Ppyyrr

Probability of ppyyRr

  P p
P PP Pp
p Pp pp

Probability of pp = 1/4

  Y y
y Yy yy
y Yy yy

Probability of yy = 1/2

  R r
r Rr rr
r Rr rr

Probability of Rr = 1/2

Probability of ppyyRr = 1/4 × 1/2 × 1/2 = 1/16


Thursday, October 28, 2010


Incomplete Dominance

Snapdragon flowers: Red (CRCR) × White (CWCW) gives 100% Pink (CRCW) in F1

F2 generation:

  CR CW
CR CRCR CRCW
CW CRCW CWCW


Co-dominance

More than two alleles control a trait (like how words can be spelled in different ways: color vs. colour)

Allele diversity comes from mutation [vocab 11]; changes in genetic sequences.

Example: Blood types

I = enzyme that attaches carbohydrates to blood cell A, B = carbohydrates

IA and IB are both dominant; i is recessive

Allele Carbohydrates
IA → A
IB → B
i → none
Genotype Blood cell appearance Phenotype
(blood group)
IAIA or IAi Type A Blood Cell.png A
IBIB or IBi Type B Blood Cell.png B
IAIB Type AB Blood Cell.png AB
ii Type O Blood Cell.png O

Multiple Genes

Epistasis [vocab 12] can turn genes either on or off. E is epistatic to B ("E controls B")


Epistasis Example: Labrador coat colors

E = ability to produce pigment
e = inability to produce pigment
B = black coat
b = brown coat

E allele controls B allele to give the following results:

eebb and eeB_ lead to yellow lab
E_bb is chocolate lab
E_B_ is black lab

Polygenetic Inheritance [vocab 13] acts as a "dimmer switch"; spectrum of phenotypes


Polygenetic Inheritance Example: Skin color

Figure 14.13 Skin Color

3 genes affect color of skin. The heterozygous individuals (AaBbCc) represented by the two rectangles at the top of this figure each carry three dark-skin alleles (black circles, which represent A, B, or C) and three light-skin alleles (white circles, which represent a, b, or c).

Punnett square shows all possible genetic combinations in gametes and offspring of a large number of hypothetical matings between these heterozygotes. The results are summarized by the phenotypic ratios under the Punnett square.

Pedigrees

Males are squares, females are circles

Affected individuals' shape is filled in, unaffected is left white, and carrier is halfway filled

Genetic Disorders

Recessively inherited
sicle-cell anemia, cystic fibrosis, Tay-Sachs, albinism
Some people can be carriers (heterozygous) for these disorders and pass them on
in order to inherit these (sometimes lethal) traits, both parents have to be carriers.
Dominantly inherited
polydactyly, Huntington's disease, Achondroplasia (dwarfism)
usually not lethal or lethal later in life. This is how they are maintained in the population


Vocabulary

  1. ↑ a true-breed plant will produce the same trait when self-pollinated
  2. ↑ the P generation is the parent generation (F0)
  3. ↑ Fn generations are the offspring of the generation before them (Fn – 1)
  4. ↑ an allele is an alternate version of a gene
  5. ↑ phenotypes are outwardly visible physical traits (does not differentiate between PP and Pp)
  6. ↑ genotypes are the allele combinations that may not necessarily be seen. Can be homozygous or heterozygous
  7. ↑ Homozygous genotypes have the same dominant/recessive allele combination (e.g. PP or pp)
  8. ↑ Heterozygous genotypes have different combination of dominant and recessive alleles (e.g. Pp)
  9. ↑ the law of segregation states that each gamete (sperm and egg) will have one allele for a given trait, i.e. the alleles for each gene will separate during gamete formation
  10. ↑ the law of independent assortment states that if the two traits we are following in a dihybrid cross (a 4x4 punnett square) are on different chromosomes, then the chromosomes carrying those alleles will randomly align along the metaphase plate during Meiosis I.
  11. ↑ mutations are changes in genetic sequences
  12. ↑ epistasis is when one gene alters/controls the presence (phenotypic expression) of another gene; acts like an on/off switch. When A controls B, we say "A is epistatic to B"
  13. ↑ polygenetic inheritance adds or subtracts the effect of other genes; acts like a dimming switch
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