Download Introduction to Genetics - Cherokee County Schools

Introduction to Genetics
Chapter 11
Gregor Mendel




Born in 1822
Was a monk and taught
high school
He loved working in the
monastery gardens
Decided to study
inheritance in pea plants
Mendel’s Pea Plants
He chose pea plants because they
reproduce quickly and produce large
numbers of offspring
 He studied seven traits with two
contrasting characters: seed shape,
seed color, seed coat color, pod shape,
pod color, flower position, and plant
height

Mendel’s Pea Plants
Some terminology…
Genetics – the scientific study of
heredity
 True-breeding – produce offspring
identical to themselves if they selfpollinate
 Hybrids – offspring of crosses
between parents with different traits

More terminology…
P generation – the parents of a cross
 F1 generation – “first filial”, the
offspring of the P generation
 F2 generation – “second filial”, the
offspring of the F1 generation

Mendel’s Experiment
Mendel crossed parents who were truebreeding with opposite traits (tall vs.
short)
 The F1 generation all had the same trait
(tall)
 He then allowed the F1 generation to
self-pollinate…

Mendel’s Experiment


…and the other trait reappeared!
¾ of the offspring had the trait of the F1
generation, but ¼ of the offspring had the
trait that disappeared
Mendel’s Conclusions
Inheritance is determined by factors
(genes) that are passed from one
generation to the next.
 These genes can come in different
forms called alleles.
 Principle of dominance – some
alleles are dominant and others are
recessive

Mendel’s Conclusions

Principle of
Segregation –
during meiosis the
alleles separate
from one another so
that each gamete
receives one allele
More Terminology…
Dominant – the trait that is expressed (T)
 Recessive – the trait that is hidden (t)
 Homozygous – has two of the same
allele (TT or tt)
 Heterozygous – has one of each allele
(Tt)

More Terminology…
Genotype – the genetic makeup of an
organism (Tt)
 Phenotype – the physical appearance
of the trait (tall)
 Monohybrid – a cross showing one trait
 Dihybrid – a cross showing two traits

Incomplete Dominance


One allele is not
completely
dominant over
another
The phenotype of
the heterozygous
organism is
somewhere in
between
Codominance
Both alleles contribute equally to the
phenotype of the organism
 Ex. In cattle red hair is codominant with
white hair; cattle with both alleles are
roan (have both white and red hairs)

Multiple Alleles
Genes that have more than two alleles
 Ex. Blood type

–
What are the 4 blood types?
 A – I AI A
or IAi
 B – IBIB or IBi
 AB – IAIB
 O – ii
Polygenic Traits
Traits that are
controlled by two
or more genes
 Ex. Height, skin
color

Gene Linkage
Mendel’s Principle of Independent
Assortment says that the alleles
separate during meiosis independent
from each other
 Thomas Hunt Morgan identified that
genes on the same chromosome would
move together – they are “linked”

Human Chromosomes

Sex chromosomes – the chromosomes
that determine the sex of the organism
–
Male – XY
½
of the sperm with carry an X, the other ½ will
carry a Y
–
Female – XX
 All

eggs carry an X
Autosomes – all other chromosomes
Karyotype

A picture of the chromosomes
Sex-Linked Genes
Genes that are found on the X or Y
chromosome
 Most sex-linked genes are found on the
X chromosome
 Since males only have 1 X, all X-linked
alleles are expressed, even if they are
recessive
 Ex. Baldness, colorblindness, hemophilia

Autosomal Disorders
Chromosomal Disorders
Down syndrome or Trisomy 21 –
caused by an extra 21st chromosome
 Turner’s syndrome – females with only
one X chromosome (XO)
 Klinefelter’s syndrome – males with an
extra X chromosome (XXY)

Pedigrees

A type of “family tree”
–
–
–
–
–
–
–
Circle – female
Square – male
Shaded – has the trait
Unshaded – does not have the trait
½ shaded – is a carrier of the trait
Horizontal line – represents marriage
Vertical line – connects parents to children
Pedigrees