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Transcript
Notes: Genetics
Mendel & Human Genetics
(Honors/Gifted)
Genetics & Heredity
 What is genetics?

the science of heredity
 What is heredity?

passing of traits from parents to offspring
Gregor Mendel
 Austrian Monk interested in inheritance
 Studied offspring from different “matings,” or
crosses, of pea plants.
 He began his research with pea plants that
were considered true-breeding.

Plants that have the ability to produce offspring
identical to themselves, through the use of their own
gametes = “Self-Pollination” = true breeding
Gregor Mendel
 Most plants reproduce via cross-pollination, the
combining of gametes from two separate plants.
 Mendel DID NOT want to have this extra variable in his
research, so he prevented his plants from “self-pollinating”
by crossing the pea plants himself.
 He began his research looking at 7 different pea
plant TRAITS
 a characteristic that varies from one organism to another
Mendel’s Research
 All 7 traits Mendel studied occurred in one of 2 forms.
 He cross-pollinated plants with different forms of the same
trait (i.e. purple x white) to examine offspring.
After crossing these plants
Mendel discovered that…
1. Inheritance is determined by genes.

Segments of DNA that determine a specific trait
2. Some alleles (different forms of a gene) are
considered Dominant and some are Recessive: The
Principle of Dominance.


Dominant alleles that are expressed (usually capital letters)
Recessive  alleles that are hidden (usually lower-case)
His Experiments
 Mendel conducted Monohybrid Crosses (mating
involving only one pair of traits).
 He cross-pollinated 2 pea plants with opposing traits
 The original mating pair he used
was called the Parental or
P – Generation.

Ex  Purple flowering plant &
white flowering plant
Mendel - Monohybrid Crosses


The offspring of this Parental cross is called the
F1 Generation, meaning “first filial.”
Results: ALL PLANTS’ FLOWERS WERE
PURPLE!
Mendel – Monohybrid Crosses
 Mendel’s 2nd Experiment…

Mendel allowed the F1 Generation (all purple) to
self-pollinate.
 Results: The F2 Generation
(“second filial”) produced flowers
in a ratio of 3 purple: 1 white
 This 3:1 ratio occurred for each
of the 7 traits in the F2 Generations!
Alas…Mendel’s Conclusions
1) Every inherited trait has 2 copies of the gene –
one from each parent.
2) There are alternative versions of genes (alleles).
3) When 2 different alleles occur together, one can
be completely expressed (dominant) while the other
can be hidden (recessive).
4) Gametes (sperm and eggs) each carry one allele
for a given trait; during fertilization, the
offspring receives 1 allele from each parent.
Now let’s review some basic
Terms…
 Gene: Segment of DNA that determines a specific
trait
 Trait: a characteristic that varies from one
organism to another
 Allele: Different forms of a gene
 Dominant Allele: an allele that hides a recessive
trait; usually characterized by a capital letter.
 Recessive Allele: an allele that can be “masked” or
hidden by a dominant allele; usually characterized
by a lower-case letter
Homozygous vs. Heterozygous
 Homozygous: Offspring have two identical alleles
(PP = homozygous dominant; pp = homozygous recessive);
also known as “pure-bred”
 Heterozygous: Offspring has one of each allele
(Pp = heterozygous); also known as “hybrid”
Genotype v. Phenotypes
 Genotype: the set/ combination of alleles an organism has
for a certain trait…the “letters” (ex: PP, pp, or Pp)
 Phenotype: the physical appearance of a trait in an
organism…. the “looks” (ex: blonde hair)
Genotype
Phenotype
The Law of Segregation
 2 alleles for a trait segregate (separate) when
gametes are formed; each offspring receives one
trait from their parents
The Law of
Independent Assortment
 Alleles for different genes separate
independently of each other during gamete
formation.
 One trait, like flower color,
does not influence the
inheritance of another trait,
like plant height.
Probability & Genetics
 Probability = the likelihood that a particular event
will occur.
 Principles of probability can be used to predict the
outcomes of genetic crosses.
 The more trials conducted, the closer the result
will come to the EXPECTED ratio.
 The Punnett square can predict the “probability” of
outcomes resulting from a genetic cross.
Predicting Results:
Punnett Squares
 Punnett Squares are diagrams that use the Laws of
Segregation and Independent Assortment to predict
offspring
 Possible gametes for 1 parent are placed along the
top of the square; possible gametes for the other
parent are written on the left of the square.
 The genotypes are predicted by combining alleles
R
r
from each parent.
R
RR
Rr
r
Rr
rr
For example…
 We can use ratios to express
genotypes and phenotypes.

Genotypic Ratio =
___ YY: ____ Yy :___yy

Phenotypic Ratio =
____ yellow : _____ green
Predicting Results:
Punnett Squares
 Genotypic Ratio =
0 YY : 4 Yy : 0 yy 
4 Yy or 100% Yy
 Phenotypic Ratio =
4 yellow : 0 green
100% yellow
Predicting Results:
Punnett Squares
 Dihybrid crosses (crosses involving 2 traits) are a
little more complex.
 Possible combinations for the different types of alleles are
placed at the top and sides of the square.
 Example: (Round, Yellow)  (wrinkled, green)
RrYy
rryy
Possible
allele
combos
RY
Ry
rY
ry
ry
RrYy
Rryy
rrYy
rryy
ry
RrYy
Rryy
rrYy
rryy
ry
RrYy
Rryy
rrYy
rryy
ry
RrYy
Rryy
rrYy
rryy
BEYOND DOMINANT &
RECESSIVE ALLELES
Part II:
Beyond Dominant & Recessive
Alleles…
 Some traits are Polygenic, meaning that
more than one gene determine the
phenotype.

Examples  human hair color, eye color,
weight, skin color
Incomplete Dominance
 One form of a trait is NOT dominant or recessive to
the other. Results in an “in-between” phenotype three different phenotypes are possible.
 Example Four O’Clocks flower
Red (RR) X White (WW)  Pink (RW)
Co-dominance
 When two dominant alleles
are expressed at the same
time in an offspring

Example: A homozygous red horse
mates with a homozygous white
horse to produce a horse with
BOTH red and white hair (called a
Roan coat).
BLOOD TYPES
Part III:
Blood Types:
A Result of Multiple Alleles
Multiple Alleles – Genes with 3 or more possible
alleles determining a trait


Each individual receives only 2 alleles, but there are
more than 2 in the population
Example = Blood Type
 Blood Phenotypes = A, B, AB, and O
 Blood Alleles = IA, IB, and i
 IA and IB are both dominant (codominant when
together), & i is recessive
Blood Typing Chart
Blood Type
Possible
Genotypes
Can
DONATE
A ;AB
Can
RECEIVE
Type A
IAIA
IAi
Type B
IB I B
IAi
B ;AB
B;O
Type AB
IAI B
AB
Universal
RECEIVER
Type O
ii
Universal
DONOR
O
A;O
Blood Type Frequency in
Population
HUMAN GENETICS
Part IV:
Human Genetics
 Sex Chromosomes The two chromosomes that
determine an individuals sex are XX (mom) or
XY (dad).
 Autosomal Chromosomes  The other 44
chromosomes, not sex chromosomes
 Karyotype = a picture of chromosomes arranged in
23 matching pairs; Sex chromosomes are ALWAYS
on pair #23.
2 Types of Disorders
 Sex-Linked Disorders
 Chromosomal Disorders
***But first, let’s talk a little about SexLinked Traits…***
Sex-Linked Traits
 All eggs carry an X chromosome
 Females are XX; Males carry XY
 In females, if a defective gene rides on one of the X
chromosomes, the other X is likely to have a good
copy of the gene that can take over for the “bad”
gene
 Males do not carry the backup copy of the X
chromosome, so the gene is expressed
Sex-Linked Disorders
 The information/ traits on the sex chromosomes are
called sex-linked genes. Because these
chromosomes determine sex, disorders caused by
these genes are sex-linked disorders. Typically,
these genes are found on the X chromosome.
 Examples



Colorblindness
Hemophilia
Duchene Muscular Dystrophy
Chromosomal Disorders
 Nondisjunction (failure of homologous
chromosomes to separate) occurs during meiosis.
The resulting individual has an abnormal number of
chromosomes and that results in a disorder!
 Examples:



Down Syndrome (Trisomy 21)
Turner’s Syndrome (XO) sterile
Klinefelter’s Syndrome (XXY)