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GENETICS
AND HEREDITY Biology Chapter 11
2/2/16
GREGOR MENDEL • Gregor Mendel is considered “The Father of
Genetics" • Austrian Monk.
• Experimented with “pea plants”. • Used pea plants because:
They were available They reproduced quickly They showed obvious differences in the traits
• Understood that there was something that carried traits from one generation to the next- “FACTOR”.
Mendel was fortunate he chose the Garden Pea
•Mendel probably chose to work with
peas because they are available in
many varieties.
•The use of peas also gave Mendel
strict control over which plants mated.
•Fortunately, the pea traits are
distinct and were clearly contrasting.
• To test the hypothesis, Mendel crossed true-breeding plants that had two distinct and
contrasting traits—for example, purple or white flowers.
• What is meant by ―true breeding?‖
**True Breeding = If mated with another identical individual, then you
will get 100% identical offspring; all the offspring will look like the
parents.
(Mendel cross-fertilized his plants by hand. Why is it important to control
which plants would serve as the parents?)
Purple (Pp)
Purple (PP)
P p p p
P
P
P
p
F1 generation All purple
White (pp)
Purple (Pp)
Pp Pp PP Gametes
F2 generation ¾ purple, ¼ white
Gametes Gametes
Gametes Pp
Pp Pp Pp
pp
MENDEL’S MONOHYBRID CROSS
MENDEL OBSERVED THE SAME PATTERN IN
CHARACTERS
Mendel’s Discovery
• Principle of Independent Assortment
• Inheritance of one trait has no effect on the inheritance of another trait
GENETICS • Genetics is the study of genes.
• Inheritance is how traits, or characteristics, are passed on from generation to generation.
• Chromosomes are made up of genes, which are made up of DNA.
• Genetic material (genes,chromosomes, DNA) is found inside the nucleus of a cell.
Traits • Genetics – study of how traits are passed from parent
to offspring
Traits • Traits are determined by the genes on the
chromosomes. • A gene is a segment of DNA that determines a trait.
Chromosomes • Chromosomes come in homologous pairs, thus genes come
in pairs. • Homologous pairs – matching genes – one from female
parent and one from male parent
• Example: Humans have 46 chromosomes or 23 pairs. • One set from dad – 23 in sperm
• One set from mom – 23 in egg
Gene for eye color (blue eyes)
Gene for eye color (brown eyes)
Homologous pair of chromosomes
Homologous Chromosomes:
Alleles – different genes (possibilities) for the same trait • ex: blue eyes or brown eyes • These are usually represented by
upper/lowercase versions of the same letter; A a.
Alleles:
• Different versions of the same gene.
• The gene for a characteristic is located at a specific position on homologous
chromosomes.
• For each characteristic, an organism inherits two alleles, one from
each parent
How do alleles differ?
Dominant allele
Recessive allele
Recessive allele
Recessive allele
Dominant - a term
applied to the trait
(allele) that is
expressed
regardless of the
second allele.
Recessive - a term
applied to a trait
that is only
expressed when
the second allele is
the same (e.g.
short plants are
homozygous for
the recessive
allele).
Dominant
• Gene that prevents the other gene from “showing” – dominant • If you have any
copies, you will see that trait
Recessive • Gene that does NOT
“show” even though it is present – recessive • Must have both
copies of it in order to see the trait
TYPES OF ALLELES
• Symbol – Dominant gene – upper case letter – T
Recessive gene – lower case letter – t
• Straight thumb is dominant to hitchhiker thumb
• T = straight thumb • t = hitchhikers thumb
(Always use the same letter for the same alleles)
Possible Phenotypes/ Genotypes • Straight thumb = TT
• Straight thumb = Tt • Hitchhikers thumb = tt
* Must have 2 recessive
alleles for a recessive trait to “show”
EXAMPLES
• When both alleles of a pair are the same – homozygous or purebred
• TT – homozygous dominant • tt – homozygous recessive
• One dominant and one recessive gene – heterozygous or hybrid
• Tt – heterozygous
TYPES OF ALLELE COMBINATIONS
BB – Black
Bb – Black w/ white gene bb – White
Genotype
• Combination of genes/alleles an organism has.
• actual gene makeup) – genotype • Ex: TT, Tt, tt
Phenotype
• Physical appearance resulting from gene /allele makeup
• Ex: hitchhiker’s thumb or straight thumb
GENOTYPE VS PHENOTYPE
PUNNETT SQUARES
• Used to predict the possible gene makeup of offspring – Punnett Square
• Example: Black fur (B) is dominant to white fur (b) in mice
1. Cross a heterozygous male with a homozygous recessive female
Bb Bb
bb bb
Write the ratios in the following orders:
Genotypic ratio
homozygous : heterozygous : homozygous dominant recessive
Phenotypic ratio
dominant : recessive
Possible offspring
Male gametes - N
(One gene in sperm)
Female gametes – N (One gene in egg)
Male = Bb X Female = bb
Genotypic ratio = 2 Bb : 2 bb
50% Bb : 50% bb
Phenotypic ratio = 2 black : 2 white
50% black : 50% white
B b
b
b
BB Bb
bb bb
B b
B
Genotypic ratio = 1 BB : 2 Bb : 1 bb
25% BB : 50% Bb : 25% bb Phenotypic ratio = 3 black : 1 white
75% black : 25% white
Cross 2 hybrid mice and give the genotypic ratio and phenotypic ratio. Bb Bb
b
INCOMPLETE DOMINANCE
• Pattern of gene expression in which the phenotype of a
heterozygous individual is intermediate (blend) between
those of the parents.
Alleles mix together and create an intermediate condition
Example: Black Fur and white fur combine to create
grey fur.
Incomplete Dominance • When one allele is NOT completely dominant over
another (they blend)
• Example: In carnations the color red (R) is incompletely dominant over white (W).
• The hybrid color is pink. Give the genotypic and phenotypic ratio from a cross between
2 pink flowers.
RW RR RW
RW WW
R
W R
W
Genotypic = 1 RR : 2 RW : 1 WW
Phenotypic = 1 red : 2 pink : 1 white
INCOMPLETE DOMINANCE
X
R1
R2
F1
F2 R2R2 does not produce pigment
Heterozygote only produces ½ pigment of R1R1
http://www.google.ca/images?hl=en&q=incoplete%20dominance&rlz=1W1SNNT_enCA397&um=1&ie=UTF-8&source=og&sa=N&tab=wi&biw=1345&bih=555
http://2.bp.blogspot.com/_uPFMFfHgGGg/SygBEtl5D1I/AAAAAAAAAKc/nylZiiL2k7s/s320/incomplete+dominance.jpg
CODOMINANCE
• The condition in which both alleles in a heterozygous
organism are expressed.
• BOTH alleles contribute to the phenotype.
• The two alleles don’t blend, but are rather both present in the
offspring.
Example: Alleles for Black Fur and White Fur would combine
to create black and white spotted fur.
• Example: In certain chickens black feathers are codominant with white feathers; so you see both colors at the same time. • Heterozygous chickens have black and white
speckled feathers. (Both black and white are present
in one individual)
http://search.vadlo.com/b/q?rel=2&keys=Dominance+Incomplete+Dominance+Codominance+PPT
Roan Horse
Roan Cattle
Variegated Clover
http://3.bp.blogspot.com/_zUPrgMM5tUg/SSVdrs59_uI/AAAAAAAAAA8/qJGjG2nZPhk/s200/codominance.GIF
http://www.ccs.k12.in.us/chsteachers/BYost/Biology%20Notes/CH11notescoincompletedom_files/image004.jpg
BLOOD TYPES
Blood Types; A, B, AB, and O
Human blood type is an example of Codominance with the A and B.
If a person receives A and B alleles from parents both will be
expressed.
O is recessive and will only be displayed if there are 2 copies.
AB = Universal Acceptor
O = Universal Donor
BLOOD TYPES
• 4 blood types
Genotype
AA or AO
BB or BO
AB
OO
Blood type
A
B
AB
O
A B
Example: What would be the possible blood types of children born to a female with type AB blood and a male with type O blood?
A O
AO BO
AO BO
O
O
Children would be type A or B only
PRACTICE QUESTIONS
1. In a certain case a woman’s blood type was tested to be AB. She
married and her husbands blood type was type A. Their children have
blood types A, AB, and B.
a. What are the genotypes of the parents?
b. What are the genotypic ratios of the children?
2. In a certain breed of cow the gene for red fur, R,
is codominant with that of white fur, W.
a. What would be the phenotypic & genotypic ratios of the
offspring if you breed a red cow and a white bull?
b. What would they be if you breed a red & white cow with
a red & white bull?
3. A rooster with grey feathers is mated with a
hen of the same phenotype. Among their offspring
15 chicks are grey, 6 are black and 8 are white.
a. What is the simplest explanation for the inheritance of these
colors in chickens?
b. What offspring would you expect from the mating of a grey
rooster and a black hen?
Dihybrid Cross
• Dihybrid cross : Examines two traits of interest at once.
• These traits are NOT linked; they are inherited seperately.
• You perform a dihybrid parental generation differs in two traits
• example-- cross round/yellow peas with wrinkled/green ones
Round/yellow is dominant
RY Ry rY ry
RY
Ry
rY
ry
What are the expected phenotype ratios in the F2 generation?
round, yellow = round, green =
wrinkled, yellow = wrinkled, green =
1 brown and curly
BBHH BBHh BbHH BbHh
BBHh BBhh BbHh Bbhh
BbHH BbHh bbHH bbHh
BbHh Bbhh bbHh bbhh
BH
BH
Bh
Bh
bH
bH
bh
bh
9 black and straight
3 black and curly
3 brown and straight
Gametes
Crossing involving 2 traits – Dihybrid crosses
• Example: In rabbits black coat (B) is dominant over brown (b) and straight hair (H) is dominant to curly (h). Cross 2 hybrid rabbits and give the phenotypic ratio for the first generation of offspring.
Possible gametes: BbHh X BbHh
BH BH
Bh Bh
bH bH
bh bh
Phenotypes - 9:3:3:1
Gametes
Gametes
100% black and straight
• Example: In rabbits black coat (B) is dominant over brown (b) and straight hair (H) is dominant to curly (h). Cross a rabbit that is homozygous dominant for both traits with a rabbit that is homozygous dominant for black coat and heterozygous for straight hair. Then give the phenotypic ratio for the first generation of offspring.
BBHH X BBHh Possible gametes: BH BH
Bh
(Hint: Only design Punnett squares to suit the number of possible gametes.)
Phenotypes:
RY Ry rY ry
RY
Ry
rY
ry
