Gregor Mendel (1822)
• Monk who worked in garden of monastery in Czech Republic.
• Used pea plants to study
heredity
• Known as the “father
of modern genetics”
Why Pea Plants?
• Many contrasting traits
• Reproduce sexually
• Crosses can be controlled
• Large # of offspring
• Short lifecycles
A little more about pea plants…
• Normally self pollinating
• Pollen (sperm) fertilizes the ovule (egg) to produce offspring.– Produce genetically identical offspring.
• Mendel used cross-pollination in his experiments.• Used pollen from one plant to fertilize another.
Genes and Dominance
• Mendel studied 7 traits of pea plants– Traits are specific characteristics – For each trait there are 2 contrasting
characteristics.
Generations
• P- parental generation; original pair
• F1- first filial generation; offspring from P generation
• F2- second filial generation; offspring from F1
Mendel’s Findings
• When he crossed P generation, F1 showed only one trait from the 2 parents.– Ex. Crossed Tall x Short short or tall
Mendel’s Conclusions
• Biological inheritance is determined by factors passed from one generation to the next.– Factors are called genes.– 2 contrasting forms of the same trait are
called alleles.• Ex. Trait is height, alleles are tall and short
Mendel’s Conclusions
• Principal of dominance- Some alleles are dominant and some are recessive.– Dominant- when allele is present organism
will always exhibit the dominant trait.– Recessive- only exhibit recessive trait if
dominant allele is not present.
Alleles
• Symbols for alleles:– A letter symbol is used for a specific trait.
• Ex- height, Tt
• Dominant- symbol is uppercase letter, T
• Recessive- symbol is lowercase letter, t
Segregation
• When Mendel crossed plants from the F1 generation traits from the P generation reappeared.
P Tall x Short all tall plants
F1 Tall x Tall some tall some short
Law of Segregation
• The 2 alleles for each trait must separate when gametes form.
– Parent only passes one
allele for each trait to its
offspring.
Probability and Punnett Squares
• The principles of probability can be used to predict outcomes of genetic crosses.
• Punnett square- diagram
which shows possible
Results of genetic crosses
Heterozygous vs. Homozgous
• Hetero different
• Homo same
• Homozygous dominant- 2 identical dominant alleles (upper case). Expresses the dominant trait.– Ex- TT, tall
Heterozygous vs. Homozygous
• Homozygous recessive- 2 identical recessive alleles (lower case). Expresses the recessive trait.– Ex- tt, short
• Heterozygous- Have 2 different alleles for the same trait. ( upper and lower case) Express the dominant trait.– Ex. Tt, tall
Genotype vs Phenotype
• Genotype- genetic make-up, letters– Tt vs tt
• Phenotype- physical
expression of the trait– Tall vs short
Sample Problem
• In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a squash plant homozygous for white is crossed with a plant homozygous for yellow what will the phenotypic and genotypic ratios be?
Sample Problem
• If 2 heterozygous white plants are crossed what will be the phenotypic and genotypic ratios?
Types of Genetic Crosses
• Monohybrid one trait– Ex. Tt x tt
• Dihybrid 2 traits– Ex- Hair and height– B= black, b=blonde, H=tall, h=short– What would be the phenotypes of a cross
between the following parents?
HhBb x HHbb
Types of genetic crosses cont.
• Ex 2- Shape and color of pea plants
R= round, r=wrinkled, Y= yellow, y=white
What would be the phenotypes of a cross between a plant that is RRYY and a plant that is rrYy?
Genetics Review
• If normal vision (N) is dominant to colorblindness (n) what are the chances that a heterozygous normal man and a colorblind woman will have a child with colorblindness?
Genetics Review
• In some flowers red flowers (R) are dominant to white flowers (r) and tall stems (T) are dominant to short stems (t). What is the genotypic ratio for a cross between the following plants:
RrTT x rrTT
DD:DR:RD:RR
Types of genetic crosses cont.
• Incomplete dominance– One allele is not completely dominant over
the other– Heterozyous phenotype is somewhere
between the 2 homozygous phenotypes.• 3 phenotypes instead of 2
– Use all capitol letters• Ex; RR= red, WW= white, RW= pink
Types of genetic crosses cont.
• Inc. Dom. Example:In snapdragons the combination of red and white flowered plants can produce a pink flowered plant.
RR= red WW= white RW= pink
What would be the phenotypes of the offspring if a red flower were crossed with a pink flower?
Types of genetic crosses cont.
• Co-dominance– Both alleles contribute to the phenotype– Ex- BB= black feather, WW= white feather,
BW= black and white spotted feathers.– Same rules as inc. dom. but both phenotypes
appear separately
Types of genetic crosses cont.
• Co-dominance sample problem
We have 2 fuzzy bunnies in class. One has black and white fur, the other is pure white. What are the genotypes of both rabbits?
Black and White= White=
What would be the phenotypes of the offspring if these rabbits mated?
Blood Type
• Human blood type is determined by the type of protein found on the red blood cells (A or B).
• Antigen- protein located
on blood cell
• Antibody- protein found in
plasma, prevent foreign
particles
Blood type
Blood type
Possible genotype
Antigen on RBC Antibodies in Blood
Type that can be received
A iAiA
iAi
A B A, O
B iBiB
iBi
B A B, O
AB iAiB A & B None AB, A, B,
O
O ii None A & B O
Blood Type Example
• What are the chances of a mother with A type blood and a father with O type blood having a child with A type blood?
Polygenic Inheritance
• Some traits are polygenic- have more than one gene coding for the trait.– Ex- skin color, eye color, height
Polygenic Inheritance cont
• 1. The weight of a fruit in a certain variety of squash is determined by two pairs of genes: AABB produces fruits weighing 4 lbs each and aabb produces fruits weighing 2 lbs each.
A. How many pounds does each dominant allele add to the total weight of an individual squash?
Human Genetics
• Chromosomes and Sex Determination– Humans have 23 pairs of chromosomes in
autosomal cells. (diploid)
• In sex cells there are only 23. (haploid)
• Sex chromsomomes are the only pair that are non-homologous.– XX female– XY Male
Human Genetics
• Ex- Cross between male and female for sex chromosomes:
– Chance of boy:– Chance of girl:
Sex-Linked Inheritance
• Some traits are linked to the X chromosome.
• Most genetic disorders are X-linked.
• Most X-linked disorders are expressed in males.
WHY???
Sex- Linked Inheritance
• Females usually act as carriers, they carry the recessive trait on one of their X chromosomes but the trait is not expressed.
Sex-Linked Inheritance
• Ex- Normal colored vision (N) is dominant to colorblindness (n). Colorblindness is an X-linked trait. – Males are colorblind more then women and
the gene is only found on the X chromosome.
What is the chance that a man with normal vision and a woman who is a carrier will have a child that is colorblind?
Autosomal Recessive
• Only expressed with homozygous recessive genotype.
• Heterozygous genotype
is carrier.
• If trait present usually
lethal disorder.– Ex- cystic fibrosis
Autosomal Recessive
• Ex-
C= no cystic fibrosis c= cystic fibrosis
A female homozygous dominant for cystic fibrosis marries a man heterozygous for cystic fibrosis. What is the probability the child will have cystic fibrosis?
Autosomal Dominant
• Expressed in homozygous dominant and heterozygous genotype.
• No carrier (Either you have it or you don’t)
• If trait is present usually
lethal.– Ex- Huntington’s Disease
Autosomal Dominant
• Ex
H= Huntington’s disease h= normal
A female who has Huntington’s disease and is heterozygous for the disorder marries a man who does not have the disorder. What is the probability that their child will have the disorder?
Pedigrees
• Chart which shows the relationships within a family.– Shows the expression of genetic traits.
• Symbols used:– Circle=female– Square=male– Half-shaded=carrier– Completely shaded=expresses trait
Pedigree
• Used to determine if disorder is autosomal dominant, autosomal recessive or X-linked.– If only expressed in males and carried in
female X-linked– If there are many carriers of both sexes
autosomal recessive.– If no carriers and expressed in both sexes
autosomal dominant