Transcript Chapter 8 Mendel, Peas, and Heredity

Chapter 8 Mendel,
Peas, and Heredity
8-1 Origins of Genetics
 Heredity
– the passing of traits
from parents to offspring
 Genetics – science that focuses
of heredity
 Gregor Mendel – considered the
father of genetics
 Why?
Mendel
 Austrian
monk who used his
math and science background to
study heredity
 Used the experiments of T.A.
Knight (studied pea plants)
 Used math to analyze his results
(1st to do so)
 Published his work in 1866
Why Peas?
They
taste good with butter
You can smash them with
mashed potatoes
Just kidding
 Has
many traits that come in
only two forms
 Can easily control the mating of
plants
 Small, grows fast, produces
many offspring
Mendel’s Classic Experiment
 His
experiment was called a
monohybrid cross (cross that
involves one pair of contrasting
traits)
 Used plants that were truebreeding in the beginning
 True-breeding – all offspring
would display one form of a
specific trait
Step 1
 Allowed
a group of plants to selfpollinate for several generations
 Why?
 To ensure they were true-breeding
 These plants would serve as the P
Generation (Parent)
 White Flowered TB Plant x Purple
Flowered TB Plant
Step 2
 Results
of P generation cross
were all Purple Flowered
Offspring
 Called the F1 Generation
 Recorded the number of plants
expressing the trait
Step 3
 Allowed
the F1 plants to selfpollinate
 Resulting offspring called the F2
Generation
 Most were purple flower plants,
some were white flowered plants
 Why?
8-2 Mendel’s Hypotheses
 For
each inherited trait, an individual
has a copy of a gene from each
parent
 There are alternative versions of
genes (allele)
 When two different alleles occur
together, one may be completely
expressed, while the other is not
observable (dominant vs. recessive)
 When
gametes form, alleles for
each gene separate
independently of one another
(haploid cells)
 Homozygous
– two of the same
alleles (TT or tt)
 Heterozygous – two different
alleles (Tt)
 Genotype – set of alleles an
organism has (TT or Tt or tt)
 Phenotype – physical expression
of a trait (tall or short)
Laws of Heredity
 Law
of Segregation – the alleles
of a gene are separated during
meiosis
 Law
of Independent Assortment –
States that a random assortment of
maternally and fraternally derived
chromosomes during meiosis results
in gametes that have different
combinations of genes
 Allele for plant height separates from
allele for flower color during meiosis
8-3 Studying Heredity
 Punnett
Squares show the probability
of a certain cross
 Remember, not actual results, only
possible results
Pedigree
 Pedigree
– a family history that
shows how a trait is inherited over
several generations
 Helpful for tracking genetic disorders
Autosomal or Sex-Linked?
 If
a trait is autosomal, it will affect
both males and females equally
 Sex-Linked traits have their allele on
the X chromosome
 Most are recessive
 Males usually exhibit the trait
because they only have one X
chromosome. Why?
 Females
get two chances to get a
dominant allele while males only get
one allele
 Females can be carriers of a sexlinked trait (if they are heterozygous)
Rules of Pedigrees
 Squares
= Male
 Circle = Female
 Line between male and female
means they are married
 Different generations happen on
different levels of the pedigree
Example of a Pedigree
 This
pedigree tracks albinism
 Albinism is a recessive trait that is
autosomal
 All genotypes are known
Example of a Pedigree
 This
pedigree tracks albinism
 Only genotype known are those
showing the recessive condition
Modes of Inheritance
 Autosomal
Dominant – males and
females can be affected, and trait
does not skip generations
 Autosomal Recessive – can affect
males and females, and may skip
generations
8-4 Patterns of Heredity
 Most
traits are not controlled by
dominant vs. recessive alleles
 When several genes control a trait,
they are called polygenic traits
 Eye color, skin color, height, and
weight are all polygenic traits
Incomplete Dominance
 When
an individual is heterozygous
for a trait, the individual displays an
intermediate form between the two
alleles
 Red Snap Dragon crossed with a
White Snap Dragon, and the
offspring are pink
Codominance
 When
an individual is heterozygous
for a trait, both alleles are expressed
at the same time
 When a homozygous white horsed is
crossed with a homozygous red
horse, the offspring is roan (red and
white hairs)
Multiple Alleles
 Traits
with three or more alleles
 Human Blood Type has three alleles,
 IA, IB, and i
Sex Linked Traits
 Traits
whose genes are located on
the X or Y chromosome
 Usually men are more likely to be
affected with diseases that are sex
linked traits
Environmental Influence
 Phenotypes
often are influenced by
the environment
 Examples of influences include
nutrition, social exposure, and
temperature
Genetic Disorders
 Sickle
Cell Anemia – autosomal
recessive condition caused by
abnormal hemoglobin molecules
 Tay-Sachs – autosomal recessive
condition that causes a defective
form of a brain protein
 Cystic
Fibrosis – autosomal recessive
condition that causes a defective
chloride-ion transport protein
 Hemophilia A – sex-linked recessive
condition that causes a defective
blood clotting factor
 Huntington’s Disease – autosomal
dominant condition that causes an
inhibitor of brain-cell metabolism