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Modified from South Dakota Data Center)

genetics

!  The scientific study of heredity, which is controlled by genes

heredity

!  Passing on of characteristics from one generation to the next -- parent to child

Genes and chromosomes

! Gene -- segment of DNA, codes for single protein, all genes together carry blueprint for organism

!   chromosome -- carries genes, extremely long molecule of DNA

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Gregor Mendel !   Born in 1822 in

Czechoslovakia. !   Became a monk in

1843. !   Taught biology and

had interests in statistics.

Mendel continued

!   After returning to the monastery he continued to teach and worked in the garden.

!   Between 1856 and 1863 he grew and tested over 28,000 pea plants

Mendel’s Peas

!  Easy to grow and to identify traits !  Quick and easy to produce many

offspring in many generations !  Selective breeding

easy

Genes and dominance

!   Trait : a characteristic, determined by one or more genes

!   Mendel studied seven of these traits, such as the color of the peas

!   He crossed plants showing contrasting traits, such as green peas crossed with yellow ones.

!   He called the offspring the F1 generation or first filial.

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What will happen when pure yellow peas are crossed with

pure green peas?

What happened when he crossed pure yellow peas

with pure green peas?

! All of the offspring were yellow.

What did Mendel conclude?

!  1st conclusion: Inheritance is determined by factors passed on from one generation to another.

!  Mendel knew nothing about chromosomes, genes, or DNA. Why?

What were Mendel’s “factors”

!  The “factors” that Mendel mentioned were the genes, which had not been discovered yet.

!  Each gene has different forms called alleles

!  Mendel’s second conclusion was that some alleles are dominant and some are recessive.

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Mendel’s second cross

!  He allowed the F1 generation to self-pollinate thus producing the F2 generation.

!  Did the recessive allele completely disappear?

!  What happened when he crossed two yellow pea hybrid (F1) plants?

Results:

!  ¾ of the peas were yellow, ¼ of the peas were green.

WHY???

Because of the Law of Segregation:

During the formation of the sex cells or gametes, the alleles separated or segregated to different gametes (pollen and egg)

But what does that have to do with the color of peas?

!  To explain, we need some definitions…

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• Genotype- the actual genes or alleles of an organism for one specific trait

• Phenotype- the physical appearance of a trait in an organism

Definitions • A Dominant allele or trait “hides”

the recessive trait in the phenotype of an individual.

• A recessive allele or trait is covered over (or dominated) by the dominant form and seems to disappear.

Definitions

• Homozygous= two alleles that are the same for a trait (Pure -- YY or yy)

• Heterozygous= two different alleles for a trait (Hybrid -- Yy)

Then what?

•  We use two letters to represent the genotype. A capital letter represents the dominant form of a gene (allele) and a lowercase letter is the abbreviation for the recessive form of the gene (allele).

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•  Example below: P=dominant purple and p= recessive white

The phenotype for this flower is violet while its genotype (if homozygous) is PP.

The phenotype for this flower is white while its genotype is pp (to be white the flower must have two of the recessive copies of the allele).

Punnett Squares !  We use a Punnett

square to work out what the possible offspring of two parents will be. l  This tool shows all the

possible combinations and the probable offspring ratios or percentages.

Punnett Square

!   Developed by Reginald Punnett.

Probability

!   The likelihood of a particular event occurring. Chance

!   Can be expressed as a fraction or a percent.

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Probability and statistics

!  No one event has a greater chance of occurring than another.

!  You cannot predict the precise outcome of an individual event.

!  The more trials performed, the closer the actual results to the expected outcomes.

Reading Punnett squares

!  Gametes are placed above and to the left of the square

!  Offspring are placed in the square. !  Capital letters (Y) represent dominant

alleles. !  Lower case letters (y) represent

recessive alleles.

Let’s set up a Punnett Square… We begin by constructing a grid of two perpendicular

lines.

Next, put the genotype of one parent across the top and the other along the left side.

For this example, let’s consider a genotype of YY crossed with yy for the yellow and green peas.

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Y Y

y

y

•  Notice only one letter goes above each box

•  It does not matter which parent’s genotype goes on either side.

Next, fill in the boxes by copying the column and row head-letters down and across into

the empty spaces.

Yy

Y Y

y

Yy

Yy

Yy y

Now, let’s explain those “reappearing” green peas. Take two heterozygous plants from the 2nd generation and cross them:

Y y

Y

y

How many of each type of pea do you get?

YY Yy

Yy yy

Y y

Y

y

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Genotype: YY: 1/4 Phenotype: Yy: 2/4 yellow: 3/4

yy: 1/4 green: 1/4

YY Yy

Yy yy

Y y

Y

y

Punnett Squares

!  Now that we have learned the basics of genetics lets walk through some examples using Punnett Squares.

For flowers:

W- dominant white

w- recessive violet

Parents in this cross are heterozygous (Ww).

Note: Make sure your capital letters are different from lowercase letters.

What percentage of the offspring will have violet flowers?

Usually write the capital letter first

For flowers:

W- dominant white

w- recessive violet

W w W

Parents in this cross are heterozygous (Ww).

Note: Make sure your capital letters are different from lowercase letters.

What percentage of the offspring will have violet flowers?

ANSWER: 25% (homozygous recessive)

Usually write the capital letter first

w

W W W w

W w w w

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Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond.

Rr rr

Rr rr

R r

r

r

What percentage of the offspring will have red hair? 50%

Let’s try some more…

In pea plants, tall pea plants (T) are dominant over short pea plants (t). Construct a Punnett Square for a heterozygous tall pea plant and a

short pea plant.

Tt tt

Tt tt

T t

t

t

What are the percentage of phenotypes?

50% tall

50% short

Black eyes (R) is dominant over red eyes (r) in rats. Make a cross between a homozygous

rat with black eyes and a rat with red eyes.

Rr Rr

Rr Rr

R R

r

r

What is the possibility of a red eye off springs?

0% L

What about 2 characteristics?

!  Mendel studied 7 traits.What did he find out when he followed 2 traits at once?

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He observed independent assortment !  Alleles

separate independently during the formation of gametes.

Mendel tried 2 factor crosses

!  For example, he followed the color and shape of peas.

!  Round yellow mated with wrinkled green -- RRYY x rryy

•  Offspring would all be hybrid for both traits (RrYy)

Try it!

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Incomplete dominance

!  A situation in which neither allele is dominant.

!  When both alleles are present a “new” phenotype appears that is a blend of each allele.

!  Alleles will be represented by capital letters only.

Japanese four-o-clock flowers

!  Red flower plant genotype = RR !  White flower plant genotype = WW !  Pink flower plant genotype = RW

What happens when a red flower is crossed with a white flower?

!  According to Mendel either some white and some red or all offspring either red or white.

! All are pink

Multiple allele inheritance

!  When two or more alleles contribute to the phenotype.

!  Human blood types: A,B,O and AB !  A and B are codominant to each other. !  Both A and B are dominant over O.

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Chromosome number

!   All cells of an organism contain a specific number of chromosomes.

!   Most cells are diploid (2n) meaning they have two copies of each chromosome

Human chromosomes

!  Humans have 23 pairs of chromosomes (46 total).

!   One from each pair comes from mom and the other from dad.

Human chromosomes !   One pair: sex

chromosomes -- determine gender:

!   XX - female XY -- male !   The other 22 pairs

are called somatic chromosomes.

Meiosis

!   An orderly method of dividing a cell’s chromosomes so that the resulting cells are haploid (have exactly 1 copy of each chromosome)

!   Meiosis produces gametes

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!  gametes join to form a zygote (very early embyo)

!   the zygote has 2 copies of each

chromosome -- one from mother and one from father

How does meiosis explain Mendel’s laws of segregation?

In the formation of gametes, each pair

of chromosomes separates; different alleles on matching (homologous) chromosomes separate.

Examples: Hh mother can pass on

either H or h, but not both

How does meiosis explain Mendel’s law of independent assortment? It is likely that 2 different genes are on

2 different chromosomes, since we have 26 pairs. The chromosomes sort independently, so most genes sort out independently as well.

Genes only sort together if they are on the same chromosome. In that case, we say they are linked.

Sex-linked

!  Genes on the X chromosome are sex-linked.

!  Sex-linked genes explain why colorblindness occurs more often in men than women.

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References ! http://www.athro.com/evo/gen/punnett.html ! http://www.kidshealth.org/kid/talk/qa/what_is_gene.html !   http://brookings.k12.sd.us/biology/ch%2011%20genetics/

punnettpractice.ppt#1 ! http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/

sciber/punnett.htm !   http://www.biotechnologyonline.gov.au/images/contentpages/

karyotype.jpg

!  Carrier – heterozygous for disease, but not affected by disease

!  Pedigree – family tree, used to trace

alleles through generations