UNIT 7
GENETICS
READING:
Chapter 8
Chapter 11
Mendel and Heredity
Gene Technology
OBJECTIVES
At the end of this unit of study the student will be able to:
1.
Define heredity and genetics
2.
Describe Mendel's contributions to an understanding of the principles of heredity.
3.
Explain Mendel's conclusions
4.
Distinguish between genotype and phenotype.
5.
Distinguish between homozygous and heterozygous
6.
Define probability
7.
Demonstrate the use of Punnett squares to find genotypes and phenotypes of
offspring of monohybrid and dihybrid crosses
8.
Describe incomplete dominance
9.
Explain sex determination and sex-linked traits
10.
Describe multiple alleles
11.
Describe gene linkage, lethal genes, and nondisjunction
12.
Describe mutations and list some causes.
ASSIGNMENTS:
1.
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4.
5.
6.
GENETICS
I.
UNIT 7
The Study of Heredity
A. HEREDITY - The passing of traits from parents to their offspring
1. Causes children to resemble their parents.
2. GENETICS - The study of heredity.
B. Gregor Mendel - the father of modern genetics
1. Austrian monk - trained in mathematics and natural sciences.
2. Work conducted over period of 8 years with common garden peas -1856 to 1865.
a. One of the first to keep careful records.
b. Applied his mathematical studies to his work.
c. Initially worked with different kinds of plants.
d. Selected peas because they:
1) Grew rapidly.
2) Produced many seed (offspring).
3) Flower structure made it easy to control POLLINATION -transfer of pollen from stamen to
pistil of a flower.
a) PISTIL - female reproductive structure - egg at base.
b) STAMEN - male reproductive structure - produces pollen that contain sperm.
c) SELF-POLLINATION - process where pollen from stamen falls on pistil of the same
flower.
d) CROSS-POLLINATION - process where pollen from stamen of 1 flower falls on pistil
of another flower on another plant.
3. Identified 7 different characteristics in his pea plants which had two contrasting forms.
a. Seed Shape - Round Vs. Wrinkled
b. Seed Color - Yellow Vs. Green
c. Seed Cover Coat - Colored Vs. White
d. Pod Shape - Inflated Vs. Constricted
e. Pod Color - Green Vs. Yellow
f. Flower Position - Axial Vs. Terminal
g. Plant Height - Tall Vs. Short
4. His experiments were different from those of earlier workers.
a. Studied only 1 trait at a time, rather than everything about the offspring at once.
b. Studied results of many matings and pooled the results - earlier workers had looked at only a
few offspring from a single mating
1) Counted 7324 peas for seed shape (F2 generation)
2) Counted 8023 peas for seed color (F2 generation)
c. Used the large number of offspring to discover definite ratios of characteristics among the
offspring.
1) 5474 Round Seed: 1850 Wrinkled Seed 2.96:1
2) 6022 Yellow Seed: 2001 Green Seed 3.01:1
5. Mendel's Experiments and Observations
a. When the plants were allowed to self-pollinate, the trait always stayed the same - called these
plants "TRUEBREEDING" or "PURE"plants; Tall plants x Tall plants produced Tall plants.
b. Removed stamens from the pure plants that produced wrinkled seeds. Dusted pistil with pollen
from plants that produced only round seeds - Called these the Parental or P1 generation
c. All of the offspring of this cross resulted in plants that had round seed - called this the First
Filial or F1 generation; Also called HYBRIDS
1) HYBRID - offspring from a cross between parents differing in 1 or more traits.
2) Working with the other traits he found that 1 trait of the parental generation always
disappeared in the F1 generation.
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d. The F1 generation plants were allowed to self-pollinate
1) Called the plants of the next generation the Second Filial or F2 generation.
2) Found that some of the F2 plants had round seed and some had wrinkled seeds.
3) Similar results were obtained in working with the other traits - always 75% of 1 trait;
25% of other trait - a 3:1 ratio.
6. Mendel's Conclusions
a. Mendel did not know anything about cell reproduction - Work based on hypothesis that
FACTORS or units carried the traits he was studying - these are called GENES today.
b. Observed that offspring of true breeding plants with contrasting traits showed the trait of only
1 parent plant - called the trait DOMINANT - trait which disappeared he called RECESSIVE
c. Observation lead to his LAW OF DOMINANCE - one form of a hereditary trait, the dominant
trait, DOMINATES or prevents the expression of the recessive trait.
d. Mendel hypothesized that the factors exist in pairs since the plants which had 1 trait could
produce seeds with the opposite trait.
e. Dominance is expressed by a capital letter - usually the first letter of the dominant trait;
Recessive trait is expressed by a small letter (same as the dominant trait)
1) For round Vs. wrinkled seed - R - dominant; r - recessive
2) Hybrid would be Rr
f. Mendel hypothesized that the paired factors separate or segregate during gamete formation - has
lead to LAW OF SEGREGATION - During gamete formation the pairs of genes responsible for
each trait separate so that each gamete contains only 1 gene for each trait.
Rr
/
R
\
r
g. During fertilization the zygote gets 1 gene for the trait from mom and 1 from dad.
h. The different forms of a gene for a trait are known as ALLELES
i. Combination of alleles or genetic makeup is the organisms GENOTYPE - Rr
The appearance of the organism regardless of its genetic makeup is its PHENOTYPE - physical
appearance - Round, yellow, etc.
j. Possible Genotypes
1) RR - pure dominant - HOMOZYGOUS DOMINANT
2) rr - pure recessive - HOMOZYGOUS RECESSIVE
3) Rr - Hybrid with 1 dominant - HETEROZYGOUS DOMINANT
II. PROBABILITY
A. The likelihood of an event occurring as expressed as a ratio or a percentage.
1. Flipping a coin - 1/2 heads; 1/2 tails
2. Cards a. Chance of drawing an ace - 4/52 or 1/13
b. Chance of drawing a spade - 13/52 or 1/4
c. Chance of drawing the Ace of Spades 1/13 x 1/4 = 1/52
B. Product Rule
1. To find the probability of 2 events occurring you multiply the individual probabilities.
a. Chance of a head - 1/2; Chance of another head - 1/2
b. Chance of 2 heads in a row - 1/2 x 1/2 = 1/4
c. Chance of 4 heads in a row - 1/2 x 1/2 x 1/2 x 1/2 = 1/16
2. Each gamete has 1/2 chance of getting a particular allele
a. Homozygous Dominant - RR: Alleles - R or R = 1/1
b. Homozygous Recessive - rr: Alleles - r or r = 1/1
c. Heterozygous Dominant - Rr: Alleles - R or r; 1/2 R; 1/2 r
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C. PUNNETT SQUARE
1. Special chart used to show possible combinations resulting from a cross of 2 organisms.
2. Put female gametes along top; male gametes along left side
3. Squares show possible genotypes of offspring -used to determine phenotype and ratio of offspring.
4. Remember this is only used to predict; it does not mean it will happen.
III. Types of Crosses
A. MONOHYBRID CROSS - involves only one set of contrasting factors for a trait
1. Cross a homozygous yellow with a homozygous green
Yellow is dominant; green is recessive
P1 YY x yy
Y Y
________
F1 all Yy heterozygous yellow
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y | Yy | Yy |
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y | Yy | Yy |
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2. Cross two of the F1 generation
Yy x Yy
Y y
________
F2 - 1/4 YY; 1/2 Yy; 1/4 yy
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Y | YY | Yy |
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y | Yy | yy |
B. DIHYBRID CROSS - involves two sets of contrasting traits at one time genes are on separate
chromosomes
1. Each gamete contains 1 allele for each trait
2. LAW OF INDEPENDENT ASSORTMENT - Alleles segregate independently of each other during
gamete formation.
C. INCOMPLETE DOMINANCE/NONDOMINANCE
1. Phenotype between dominant and recessive trait
2. Heterozygous condition
3. Example - Four-O-Clocks
a. RR - red flowers
b. rr - white flowers
c. Rr - Pink flowers
4. Cross of 2 pink flower plants - Rr x Rr
a. Results - 1/4 red (RR); 2/4 (1/2) Pink (Rr); 1/4 white
D. Sex Linked Traits
1. Determination of Sex
a. By the SEX CHROMOSOMES - x or y - other chromosomes are called AUTOSOMES
b. Male - xy; Female - xx
c. Get 1/2 males and 1/2 females on Punnett square
x
y
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x | xx | xy |
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x | xx | xy |
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2. SEX LINKED CHARACTERISTICS
a. Recessive trait linked with a certain sex - usually males
b. Carried on the x-chromosome; since male has only 1 x, the trait is visible - females with 2 x's
- not visible if 1 of the x has a dominant gene for the trait.
c. Only way for female to show the trait is to be homozygous recessive.
d. Woman is called a "CARRIER" in the heterozygous condition.
e. Examples
1) Red-Green color blindness
2) Hemophilia
E. MULTIPLE ALLELES
1. More than 2 alleles exist for a particular trait
2. In humans - blood types is an example of multiple alleles
a. 3 different alleles - A, B, O
b. Alleles A and B are codominant while O is recessive.
c. Blood type is due to presence of antigens on the red blood cells - produces antibodies in blood
1) Type A - Antigen A on cells; Plasma contains anti B
2) Type B - Antigen B on cells; Plasma contains anti A
3) Type O - No Antigens; Plasma has anti A and anti B
4) Type AB - Antigens A and B on cells; lacks anti A and anti B
d. Phenotypes and genotype combinations
1) Type A - AA or AO
2) Type B - BB or BO
3) Type AB - AB
4) Type O - OO
Blood
Donor to
Receives from
Group
UNIVERSAL DONOR
O
O, A, B, AB
O
A
A, AB
O, A
B
B, AB
O, B
UNIVERSAL RECIPIENT AB
AB
O, A, B, AB
IV. Other Related Topics
A. GENE LINKAGE
1. Concerned with the presence of 2 different genes on the same chromosome
2. Does not follow usual dihybrid results - follows monohybrid.
3. Variation can occur due to crossing over - pieces of chromatids exchange places during synapsis of
tetrads in meiosis.
B. LETHAL GENES
1. Genes which can cause death or harm in the homozygous condition.
2. Examples
a. Sickle-cell anemia
b. PKU
c. Tay Sachs - Jews of middle eastern European origin.
d. Diabetes mellitus
C. NONDISJUNCTION
1. Failure of chromosomes to segregate properly during gamete formation.
2. Can involve sex chromosomes as well as the autosomes - zygote gets an improper number of
chromosomes
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3. Examples
a. Down's Syndrome - three #21 chromosomes - autosomal
b. Turner's Syndrome - has only 1 x, no y chromosome - female
c. Klinefelter's Syndrome - xxy - male
d. Jacob’s Syndrome- xyy - male – were thought to show criminal behavior at one time; not higher.
D. MUTATIONS
1. Change in the genetic code or genes of an organism.
2. Can occur naturally or by exposure to agents that produce mutations.
a. Breaks during crossing over which do not reattach.
b. Increased chance of breakage by exposures to mutagens
1) MUTAGENS - agents that cause mutations.
2) Radiation - x-rays, UV, gamma
3) Chemicals
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