Biology Chapter 9 Fundamentals of Genetics
Jan 04, 2016
Biology
Chapter 9
Fundamentals of Genetics
What is Genetics?
a. Study of heredityb. Transmission of traits from parent to
offspring
Who is the father of genetics?
• Gregor Mendel
• 1822-1884
Why was Mendel Successful?
• 2 reasons1. He used a garden pea as his test subject
• Why would he use the pea?• 6 reasons
a. Smallb. Easy to growc. Produce many offspringd. Mature quicklye. Many varietiesf. Easy to fertilize
» Self fertilization – within same plant» Cross fertilization – involved two plants
Second reason?
2. He used a quantitative approach
Mendel’s Experiment
• Step 1– He produced a parent generation (P)
• He allowed pea plants to self fertilize for many generations
• This made sure that he had pure parents that were true breeding or pure
• Example: He has 1 pure purple pea plant and 1 pure white pea plant for his P generation
Mendel’s Experiment Cont.
• Step 2– He produced the 1st generation (F1)– He cross fertilized two of the P generation pea
plants– What was his results?
• 100% Purple plants, no white
Mendel’s Experiment Cont.
• Step 3– He produced the 2nd generation (F2)– He took 2 of the F1 generation pea plants and
self fertilized them.– What were the results?
• 75% purple, and 25% white
Mendel’s Conclusions
• Parents transmit information about traits to their offspring
• Each individual has 2 factors (genes) for each trait, 1 from each parent
• Factors (genes) are represented by letters or alleles.
Alleles
• If both alleles are the same, the individual is homozygous for that trait
• If both alleles are different, the individual is heterozygous for that trait
Traits can be described in 2 ways:
• Genotype– Alleles that represent the trait– Example: PP, Pp, pp
• Phenotype– Expression of the trait– Example: purple, white
More about traits
• Only some traits are seen, others are masked– Dominant – only need 1 letter to be expressed
» Represented by capital letters
– Recessive – need both letters or its masked» Represented by lowercase letters
Do you understand?Genotype Dom/Rec Homo/Hetero Pheno
Mendel’s Law of segregation
• Members of each pair of alleles separate when gametes are formed. A gamete will receive 1 allele of the other. This occurs in meiosis.
Principle of Independent Assortment
• Two or more pairs of chromosomes separate independently of one another during the formation of gametes. This is random.
Why did Mendel’s results repeat?1. Chance and probability- Leads to predictions
1. coins- flipping a head?
2. cards- diamond?- nine?- nine of diamonds?
3. sex of children- having a boy child?- having a girl after having 4 boys in a row?
Monohybrid Crosses
• Involves 1 trait
• Crosses 2 alleles on the same locus
• Uses a 4-boxed Punnett Square
• Example: Cross a white flowered pea plant with a heterozygous purple flowered pea plant
Monohybrid Example
• pp x Ppp p
P
p
Genotype %
50% Pp
50% pp
Phenotype %
50% purple
50% white
How do you find out whether an individual is BB or Bb?
• To a testcross– Technique that takes the unknown genotype and cross it
with a recessive individual and then look at the results.
Dihybrid Crosses
• Involves 2 traits• Crosses individuals with 4 alleles at 2 loci• Uses a 16 box punnett square• When both genotypes for both individuals are
heterozygous (BbTt x BbTt), the phenotype percentage will be 9:3:3:1
• Example: Cross a homozygous purple flowered, heterozygous green pod pea plant with a white flowered, yellow pod pea plant. (Green is dominant over yellow)
Dihybrid Example
• PPGg x ppgg• Must do Foil to get the gametes!
– First, Outer, Inner, Last• PPGg• F = PG• O = Pg• I = PG• L = Pg
– FOIL for ppgg are pg, pg, pg and pg– So now take these gametes and place them
in the Punnett Square
Dihybrid ExampleDihybrid Example
Genotype: 50% PpGg, 50% Ppgg Phenotype: 50% Purple Green
50% Purple Yellow
Dominant Recessive relationships
1. Lethal recessive – homozygous recessive organisms cannot survive (ex. Tay Sachs, Cystic Fibrosis)
2. Incomplete Dominance
3. Codominance
Incomplete Dominance
• Heterozygote is an intermediate between phenotypes of two homozygotes
• Blending occurs!• Occurs in Japanese 4:00 plants and
snapdragons– RR = red– WW = white– So RW = pink!
• Example: Cross a red flowered Japanese 4:00 plant with a white flowered 4:00 plant
Incomplete Dominance Example
Genotype:
100% RW
Phenotype:
100% pink
Codominance
• Two traits share dominance (Ex. Human Blood Types)– Must use special notations when doing these
problems• IAIA and IAi…………bloodtype A• IBIB and IBi…………bloodtype B• IAIB………………….blood type AB• ii……………………..bloodtype O
• Cross a person with bloodtype AB with a person with bloodtype O
Codominance Example
IAIB x ii IA IB
i
i
IAi IBi
IAi IBi
Genotype %
50% IAi
50% IBi
Phenotype %
50% bloodtype A
50% bloodtype B
X linked Genes
• Also called Sex linked genes
• Genes that follow the transmission of the X chromosome
• Always expressed in males, and is considered to be dominant
• Females may be expressed
• Hemophilia is an example – special notation for these types of problems
Sex linked notations
• For hemophilia– Females
• XHXH normal• XHXh carrier/heterozygous• XhXh has hemophilia
– Males• XHY normal• XhY has hemophilia
Example: Cross a hemophiliac man with a female carrier
Sex Linked ExampleXHXh x XhY Xh
Xh
XH
Y
XHXh
Genotype
25% XHXhXHY
25% XHYXhXh XhY 25%XhXh
25% XhY Phenotype: 50% normal, 50% hemophiliac