Transcript Mendelian Genetics

Mendelian Genetics
Introduction to Genetics
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Introduction to Genetics
GENETICS – branch of biology that
deals with heredity and variation of
organisms.
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Chromosomes carry the
hereditary information
(genes)
Arrangement of nucleotides
in DNA
DNA - RNA - Proteins
Gregor Mendel
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Gregor Johann Mendel
Austrian Monk, born in what is
now Czech Republic in 1822
Son of peasant farmer, studied
Theology and was ordained
priest O.S.A.
Went to the university of Vienna,
where he studied botany and
learned the Scientific Method
Mendel’s Peas
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Mendel looked at 7 traits in pea plants
Mendel
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Mendel was the first biologist to use Mathematics – to
explain his results quantitatively.
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Mendel predicted
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Described the concept of genes
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Determined that genes occur in pairs
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Described that one gene of each pair is
present in the gametes (one from the sperm and one from the
egg.
Important Terminology
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Homozygous – having identical
genes (one from each parent) for a
particular characteristic (TT, tt) .
Heterozygous – having two
different genes for a particular
characteristic (Tt)
Dominant – the allele of a gene
that masks or suppresses the
expression of an alternate allele;
the trait appears in the
heterozygous condition. (Tt, TT)
Recessive – an allele that is
masked by a dominant allele; does
not appear in the heterozygous
condition, only in homozygous. (tt)
Check for Understanding #1
Terminology Cont’d
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Phenotype – the physical appearance
of an organism.
Genotype – the genetic makeup of an organisms
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Gamete – a specialized sex cell that contains HALF of the
genetic information necessary for expression of a trait
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Monohybrid cross: a genetic cross involving a single pair of
genes (one trait); parents differ by a single trait.
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P = Parental
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F1 = First filial generation; offspring from a genetic cross.
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F2 = Second filial generation of a genetic cross
Monohybrid Cross
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Parents differ by a single trait.
Crossing two pea plants that differ in stem size, one
tall one short
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T = allele for Tall
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t = allele for dwarf
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TT = homozygous tall plant
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t t = homozygous dwarf plant
Punnett Square
A useful tool to do genetic crosses
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For a monohybrid cross, you need a square divided by
four….
We use the Punnett square to
predict the genotypes and
phenotypes of the offspring.
Using a Punnett Square
Steps:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a p-square
Parent genotypes:
TT and t t
Cross
TT ´ tt
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Using a Punnett Square cont’d
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4. "split" the letters of the genotype for
each parent & put them "outside" the psquare
5. determine the possible genotypes of
the offspring by filling in the p-square
6. summarize results (genotypes &
H
h
phenotypes of offspring)
H
HH
Hh
h
Hh
hh
Mendels Principles
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Principle of Dominance:
One allele masked another, one allele
was dominant over the other in the F1
generation.
Principle of Segregation:
When gametes are formed, the pairs of
hereditary factors (genes) become separated, so
that each sex cell (egg/sperm) receives only one
kind of gene.
Real Life Case: Cystic Fibrosis
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IF two parents carry the recessive gene of
Cystic Fibrosis (cf), that is, they are
heterozygous (Cf cf), one in four of their
children is expected to be homozygous for
cf and have the disease:
Cf
Cf Cf = normal
Cf cf = carrier, no
symptoms
cf cf = has cystic
fibrosis
cf
Cf
CfCf
Cfcf
cf
Cfcf
cfcf
Dihybrid Cross
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Dihybrid crosses
Matings that involving parents that differ in two
genes (two independent traits)
For example flower color
A = purple (dominant)
a = white (recessive)
and stem length:
B = tall
b = dwarf
Dihybrid Cross Ratios
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1
2
2
4
1
2
1
2
1
AABB
AABb
AaBB
AaBb
AAbb
Aabb
aaBB
aaBb
aabb
Tall; Purple Flowers (9)
Tall; White Flowers (3)
Dwarf; Purple Flowers (3)
Dwarf; White Flowers (1)
Principle of Independent
Assortment
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Based on these results, Mendel postulated the
Principle of Independent Assortment:
“Members of one gene pair segregate
independently from other gene pairs during
gamete formation”
Genes get shuffled – these many combinations
are one of the advantages of sexual
reproduction
Incomplete Dominance
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Snapdragon flowers
come in many colors.
If you cross a red
snapdragon (RR) with a
white snapdragon (rr)
You get PINK flowers
(Rr)
Incomplete Dominance
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When F1 generation (all
pink flowers) is self
pollinated, the F2
generation is 1:2:1
red, pink, white
Other Examples
•Hair Texture: Curly hair is incompletely dominant (HH) to
straight hair (SS). Those who have wavy hair are
intermediate (HS).
• Inter-eye Distance: Close-set eyes are incompletely
dominant (DD) to eyes set far apart (FF). Medium-set eyes
are DF
•Lip Protrusion: Protruding lips are incompletely dominant
(PP) to nonprotruding lips (NN). Slightly protruding lips are
intermediate (PN).
Codominance
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Two alleles that are equally
dominant
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One allele that is recessive to
both
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When both dominants are
present, both show, not a midrange.
Roan coat color in ungulates
(cows!!)
-giving both brown
and white HAIRS!
Codominance in Human Blood
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Two alleles are expressed (multiple
alleles) in heterozygous individuals.
Example: blood
1. type A = IAIA or IAi
2. type B = IBIB or IBi
3. type AB = IAIB 4. type O
= ii
Polygenic Traits
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More than one allele exists within a
population
Example: human blood groups A, B, AB,
O
Summary of Genetics
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Chromosomes carry hereditary info (genes)
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Chromosomes (and genes) occur in pairs
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New combinations of genes occur in sexual reproduction
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Monohybrid vs. Dihybrid crosses
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Mendel’s Principles:
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Dominance: one allele masks another
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Segregation: genes become separated in gamete
formation
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Independent Assortment: Members of one gene pair
segregate independently from other gene pairs during
gamete formation