Introduction to Genetics

Ch 9

“Animalcules”- tiny people in sperm?

Mendel’s Laws of Heredity Hereditary factors do not combine, but are passed intact

(Law of Segregation) Each member of the parental generation transmits only

half of its hereditary factors to each offspring (with certain factors "dominant" over others) (Law of Dominance)

Different offspring of the same parents receive different sets of hereditary factors (Law of Independent Assortment)

Laws of Inheritance

Gregor Mendel (1823-1884) Augustinian monk Mendel was the first person to trace the

characteristics of successive generations of a living thing (pea plants).

So What REALLY Happens? The number of

chromosomes divide to make gamete cells.

When does this occur?

Before sexual reproduction

Sex Chromosomes

Fertilization

Law of Segregation Two members of a gene pair segregate

(separate) from each other during the formation of gametes.

What is a gene pair? When, in meiosis does segregation occur? Anaphase I

Law of Independent Assortment Alleles for different traits assort (segregate)

independently of one another. Genes on different chromosomes behave

independently in gamete production. Why does this law not apply to a one trait

cross? When, in meiosis does this occur?

Law of Independent Assortment:

The Work of Gregor Mendel A. The branch of biology that studies heredity is

called genetics. B. Gregor Mendel is considered the father of

genetics who studied in a monastery. C. He planted hundreds of pea plants to study

their traits. D. He self-pollinated plants: 1. He brushed the pollen of one plant onto

itself

2. These parents inherited all the characteristics of the single plant that bore them.

E. He also cross pollinated plants. 1. He brushed the pollen of one plant

onto another.

2. He discovered that purebred tall

plants would only produce tall offspring. F. He studied many traits. 1. Other characteristics of the pea

plants are:

Round peas or Wrinkled Yellow peas or Green Gray peas or White Smooth pods or Constricted Green pods or Yellow Axial or Terminal Tall or Short

I. Dominance vs. recessive traits: 1. Dominant traits show through with one

parent’s dominant trait: Ex. Tt = Tall 2. This is known as a hybrid or heterozygous

(two different traits) T= Dominant for tallness t= Recessive for shortness 3. An organism will always show the

dominant trait.

4. Recessive traits show through only when an organism has two recessive traits.

5. Ex. tt = short 6. TT or tt are considered purebred or

homozygous because both genes from the mother and the father are the same.

7. Phenotype: The physical appearance of the offspring in a punnett square.

8. Genotype: The genetic makeup of the offspring.

J. Mendel’s experiment: F1 generation (segregation)

He crossed a purebred for tallness with a purebred for shortness:

TT x tt

Mendel’s Experiment

The F2 generation would be Tt xTt

Do the following cross: Cross a hybrid for tallness with a purebred

for shortness: Tt x tt

Do the following cross: Two parents are hybrid for brown hair.

What are the probabilities of their offspring having brown or blond hair?

Review

Test Crosses SS x SS - true breeding (100%

homozygous dominant.) ss x ss - true breeding (100% homozygous

recessive.) How do you determine whether an

individual with the dominant phenotype is homozygous or heterozygous?

Test Cross Used to detect heterozygous condition. In peas, the allele for tall plant height (T) is

dominant over the allele for short plant height (t) What cross would you do to determine the

genotype of a tall pea plant? What results would you expect if the tall plant is

TT? What results would you expect if the tall plant is

Tt?

What would the gametes of a plant of genotype SsYy have?

SY, Sy, sY, and syEach gamete will receive either an S or s allele, and either a Y or y allele.

Following a SsYy x SsYy cross, what fraction of the offspring are predicted to have a genotype that is heterozygous for both characteristics?

Probability Probability = the # of times an event is expected to happen /

the # of opportunities for an event to happen. For example, The probability of rolling a 2 on a dice in one

trial is:1/6

The rule of multiplication states that the probability of two or more events happening at the same time is the product of the independent events.

What is the probability of rolling two 2’s?(1/6) x (1/6) = 1/36

How can the principles of probability be used to predict the outcomes of genetic crosses?

Ratio:

9:3:3:1

Cross a homozygous running, heterozygous black mouse with a waltzing brown mouse B=black and b=brown R=running and r=waltzing

Cross a waltzing, brown mouse with a waltzing, brown mouse

Cross a heterozygous running, brown mouse with a heterozygous running, homozygous black mouse

Deep down in the Pacific Ocean in the city of Bikini Bottom, square pants (S) are dominant to round pants (s), and yellow skin (Y) is dominant to green skin (y). Sponge bob is a Yellow skinned, square pants sponge, whose mother had green skin and round pants.

A. What is Sponge Bob’s genotype? B. Sponge Bob wants children and he

mates with a spongey woman with square pants and green skin. Her father had curved ears. What is her genotype?

C. What is the phenotypic ratio of the children that are expected as a result of this mating?

D. How many of their children will have the following genotype?

PPYy? Ppyy?

A man with brown eyes marries a woman with blue eyes. The mans mother had blue eyes. What is the probability that they will have a blue-eyed child? 

B= brown     b= blue You do not immediately know the genotype of the brown-eyed man

because he could be either Bb or BB.  Begin to state the cross by using a blank for the unknown gene.  One of the genes of the brown-eyed man must be B or he would have blue eyes.  His genotype is B_ and the cross is: B_ x bb --> .  Fill in any blanks in a problem by using other information in the problem.  The brown-eyed man had a blue-eyed mother.  Her genotype must be bb (If she had even one B gene she would have brown eyes).  The man's mother could give him only a b gene so his genotype must be Bb.  The cross now is: Bb x bb -->.  Do the cross. 

Ie. RrYy x RrYy

What is the probability of obtaining an offspring with the genotype rrYY?

Genetics Quiz