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Our Main Man Mendel!• Gregor Mendel father of
genetics; discovered trends in heredity 30 years before other scientists!
• Prior to Mendel what did people believe about inheritance?– Blending Theory all genetic
traits are equally mixed in offspring
– If this were true what would disappear?• Outliers (really tall, short,
etc… members of a population)
Mendel’s Work• Secret to Mendel’s discovery
is his detailed record keeping and attention to the controlled experiment
• What was his model species?– Garden peas
• What did this species work so well?– Easily grown; has set traits
(characteristics) that are easily to observe; character differences mostly in twos (white flowers vs. purple; tall vs. short)
• If you are going to observe trait inheritance, what must you be pretty sure of?– The genetic make up of
your subjects• Pollination- transfer of pollen
(male gametes) to the female reproductive organ in plants– To make pure-breeds for
each trait• Cross-pollination- using pollen
from a plant with different traits to test the trait inheritance patten
Mendel’s Monohybrid Crosses• Wanted to produce specific
hybrids of plant with different traits – Tall X Short– Purple Flowers X White
Flowers– Green Seeds X Yellow Seeds
• Started with P1
– Parents• Offspring were call F1
– Filial (son/daughter)– 1st generation
• F1 X F1 = F2
– 2nd generation
Controlling Heredity• Each organism has two factors that control
each of its traits.
– Genes exist in alternative forms. We call
these different gene forms alleles
– Alleles are located in the same locus
(area) of homologous chromosomes
(chromosome sets)
– Allele that determines the appearance
(phenotype) is dominate
– Other allele is recessive • Genotypes :
– Homozygous dominate AA– Homozygous recessive aa– Heterozygous Aa
Mendel’s Three Theories• From the first cross all F1 flowers are
purple• From F1 cross 3 purple: 1 white flower • Theories1) The parents must carry a pair of
factors (alleles) the control the inheritance of each trait– P purple flowers– p white flowers
2) If you have one of each allele, one will be dominate and one recessive– Pp purple flowers
3) Pairs of alleles are segregated into different gametes– Law of Segregation
• Gamete has either P or p
Punnett Squares• Reginald Punnett; English
biologist
• Monohybrid cross
following the crossing of
parents while focusing on 1
trait
• A cross with 1 homozygous
dominate and 1
homozygous recessive
makes what?
• Only heterozygous
offspring
• A cross between 2
heterozygous produce
what?
• ¼ TT, ¼ tt, and ½ Tt
Monohybrid Crosses • Punnett Square method for organizing
alleles during breeding that uses statics – All Statics probabilities are between 0-1
• 0impossible• 1 completely possible
• Product Rule events where both A and B will occur– Chance of heads vs. tails 0.5– Chance of heads twice 0.5x0.5= 0.25
• Sum Rule when two or more possible ways exist to get the same outcome– Chance of getting head and tail in 2
tosses 0.5x0.5=0.25 – Two ways (head and tail or tail and
head) 0.25+0.25= 0.5
Practice Crosses1) What are the possible genotype ratios
for a cross between homozygous dominate (purple) and homozygous recessive (white) garden pea plants? What is their phenotypes?– 1.0 chance of heterozygous (Pp)– All purple flowers
2) What are all possible genotype ratios for a green seed X yellow seed? Green is dominate.– 1.0 chance heterozygous (Pp)– 0.5 chance heterozygous (Pp) and 0.5 homo
recessive (pp)– All green seeds; half green and half yellow
seeds
P P
p Pp Pp
p Pp Pp
G G
g Gg Gg
g Gg Gg
G g
g Gg gg
g Gg gg
Genetic Terminology • What is a genotype?
– Allele pairing in genes• What is a phenotype?
– Physical appearance of alleles• Word for AA genotype?
– Homozygous dominant• Word for aa genotype?
– Homozygous recessive• Word for Aa genotype?
– Heterozygous• What do you call a cross between
two subjects that follows the inheritance of one trait?– Monohybrid Cross
Dihybrid Cross• Cross following two traits
with both subjects being heterozygous for each trait (AaBb X AaBb)
• What ratio does a dihybrid cross always produce?– 9:3:3:1– 9 AB; 3 Ab; 3 aB; 1 ab
• What does this ratio tells us about alleles for A and B?– They each sort randomly into
gametes (not linked)– Principle of Independent
Assortment
Practice Crosses• What is ratio of the
phenotypes of a RRYy x Rryy?
• R round seed• r wrinkled seed• Y yellow seed • y green seed
– 1.0 round seed; 0.5 yellow and 0.5 green seed
• What phenotype will not be present after a Rryy x rryy cross?– Yellow seeds
RY RY Ry Ry
Ry RRYy RRYy RRyy RRyy
Ry RRYy RRYy RRyy Rryy
ry RrYy RrYy Rryy Rryy
ry RrYy RrYy Rryy Rryy
Ry ry Ry ry
ry Rryy rryy Rryy rryy
ry Rryy rryy Rryy rryy
ry Rryy rryy Rryy rryy
ry Rryy rryy Rryy rryy
Chromosome Theory of Inheritance• Sum up what we have learned from
Mendel:1) Chromosomes occur in pairs in
sexually reproducing, diploid organisms. Alleles for each gene are on these chromosomes
2) The chromosomes of each pair are separated and delivered to different gametes. This also separates alleles of genes
3) The separation of any pair of chromosomes in meiosis and gamete formation is independent
4) One member of each chromosome pair after fertilization comes from the male and female parents
Alternative Inheritance Patterns1) Incomplete dominance– Phenotype for heterozygous
genotype is a blend of both homogenous traits
– Recessive trait is not completely blocked because dominate trait is incomplete
– CRCR x CWCW give what type of offspring ratios?• 100% pink (heterozygous)
– What about F2?• 1 Red: 2 Pink: 1 White
Alternative Inheritance Patterns2) Codominance– Both alleles are expressed
equally and produce equal effects in heterozygotes
– MN blood groups• People either have M form
glycoproteins or L form glycoproteins• If you get both alleles you
have both glycoproteins
– Same inheritance patterns as incomplete dominance
Alternative Inheritance Patterns3) Multiple Allele Inheritance– Several types of same allele
will code for different phenotypes
• Human Blood-Types– Allele I-> IA, IB, or i IAIA IBIB IAIB ii
Alternative Inheritance Patterns4) Epistasis gene interaction; genes on one allele hide the phenotypes of genes on another allele– Common in skin tones and
animal colors• B and C allele produce dark
pigments while b and C produce less/no pigment
• B and C needed to make black• C needed to make brown if b• No C means white regardless
of B or b
Alternative Inheritance Patterns5) Polygenic Inheritance– Several genes contribute to
the same trait (quantitative trait); skin color, height, weight, etc…
– Typically shows bell-curve distribution in a population
Sex Chromosomes• Sex determination
– chromosomes, which
determine the sex of an
individual, are called sex
chromosomes
– 22 other chromosomes
are called autosomal
chromosomes
Sex-Linked Inheritance• Drosophila eye color:• “Wild type” Red, w+w+• Mutation White, ww• R dominate over r• What is the F1 genotype?• heterozygous; w+ w• What is the F2 ratio?• 3:1 1 w+w+, 2 w+ w, 1 ww• F2 only produces males with white eyes• Sex-linked inheritance
Sex-linked Inheritance• Y Chromosome– Sex-determining genes; SRY gene
makes females into males while an embryo
– Maybe fading from existence; may be getting smaller
– XY heterogametic • X Chromosome– Mostly codes for non-sex related traits
(ex. Color vision)– XX homogametic
• Colorblindness• Hemophilia– Low platelet number makes it difficult
to stop bleeding– Influences a revolution in Russia
Too Many Xs!• Why do females need two Xs?
– They Don’t! Two X chromosomes would mean double the genetic material necessary
• What does the body do with the X chromosome?– It randomly shuts one X down– Creates a Barr body dense mass of
inactive chromatin– They are copied and passed on in
mitosis but are never used for proteins• How can this show us X-recessive
traits?– Dominate X might be randomly
deactivated so the X recessive is randomly present in cells
– Female calico cats have a mix of orange and black fur but males are always black or orange
Environmental Effects
• Environmental Factors:– Temperature, nutrition, light, etc… can effect the
phenotype of an individual by changing the expression of the genes
– Arctic animals are influence by cold
• Internal Factors:– Hormones regulate gene expression so a genotype may
not express the same phenotype it different individuals
– Peacock color is not sex-linked, but the gene that regulates the color gene is