I can explain how genes pass traits from
parents to offspring
 I can describe the role of chromosomes
in heredity
 I can identify patterns of heredity in
humans
 I define genetics, dominant gene,
recessive gene, genotype, and
phenotype

Experiments
Human Heredity
Twins
Mendel
Morgan
Applied Genetics
Genetic
Diseases
Environment
Selective
Breeding
Genetic
Engineering

Genetics is the study of heredity

What is heredity?
› The passing on of traits from parents to
offspring
› All organisms pass information (DNA) about
traits to their offspring

More than 100 years ago, a scientist
named Gregor Mendel made important
discoveries about heredity

He took seeds from tall pea plants and
planted them.

Some plants were tall, but some were
short. WHY??
Mendel decided to study pea plants
and their seeds
 He wanted to grow only tall pea plants
 Since the flowers of pea plants have
both male and female sex organs,
Mendel tried self-pollination
 Self-pollination occurred when Mendel
moved pollen from the male sex organs
to the female sex organs on the same
plant

After self-pollination occurred, each
flower produced seeds
 Mendel planted the seeds and then selfpollinated the plants that grew from
those seeds
 He did this again and again
 Finally, Mendel had seeds that produced
only tall pea plants (pure tall seeds)


Using the same self-pollination process,
Mendel grew only short plants as well
(pure short seeds)

Mendel called these plants the P
Generation.

P stands for the parent generation
Eventually, Mendel wanted to know
what type of offspring two different pure
parent plants would make
 He used the process cross-pollination to
find out.
 Cross pollination is the movement of
pollen from the male sex organs to the
female sex organs of a different plant

So Mendel moved the pollen from pure
tall pea plants to the female sex organs
of pure short pea plants (and vice versa)
 When the seeds produced plants,
Mendel took the seeds from those
flowers and planted them.
 He called these plants the F1 Generation
 F stands for filial (son or daughter)


Mendel found that all F1 Generation
plants were tall

He crossed hundreds of pure short plants
with pure tall plants
All the results were similar…tall plants
 What happened to the short trait??

Short pure
P generation
T
T
t
Tt
Tt
(tall)
(tall)
t
Tt
Tt
(tall)
(tall)
Tall pure
P generation
F1 Generation
All tall

Mendel used these to explain his crosses

A gene is the information that a parent
passes to its offspring for a trait

A pea plant inherits two genes for height
(one gene received from each parent)

A gene that is hidden when it is
combined with another gene is called a
recessive gene (lower case letter)

The gene that shows up is called the
dominant gene (capital letter)

All pea plants were tall, so the dominant
gene is tallness
An organism’s combination of genes for
a trait is called its genotype
 For example, the genotype of the F1 pea
plants is Tt

What an organism looks like as a result of
its genes is its phenotype
 The phenotype of the F1 pea plants was
tall

Mendel then self-pollinated the F1
Generation plants
 Mendel found that short plants started
showing up again (25% of the time)
 This F2 Generation always included short
plants
 Short plants reappeared because in the
F2 Generation, the plants inherited two
recessive genes for shortness

T
F1 Generation
(Tt)
t
T
t
TT
Tt
(tall)
F1 Generation
(Tt)
(tall)
F2 Generation
Tt
tt
(tall)
(short)
¾ Tall
¼ Short

Mendel concluded that there was
information in a plant that caused it to
have certain traits

The dominant genes hid the
appearance of recessive genes

The offspring receive half their genes
from one parent and half from the other
Genetics
 Self-pollination
 Cross-pollination
 Dominant Gene

Genotype
Phenotype
Gene
Recessive Gene
Self Check questions, pg. 238 #1-5
 COMPLETE SENTENCES


Rod-shaped structures made of proteins
and a chemical called DNA

Found in the nucleus of the cell

A chromosome can contain hundreds of
genes, which determine all the traits of
an organism

Basic steps to Mitosis:
› Chromosomes make a copy of themselves
› Nuclear membrane dissolves
› The two sets of chromosomes separate
› A nucleus forms around each set of
chromosomes
› RESULT: Two new identical cells each with a
nucleus and set of chromosomes

Following mitosis, the entire cell divides

Each new cell gets one nucleus with a
complete set of chromosomes

Each set of chromosomes is identical to
the parent’s (new cell is identical to
parent cell)

Two gametes (sex cells) join to form one
complete cell

Each gamete has only half of the
chromosomes of other cells in the body

When gametes join, they form a cell that
has a complete set of chromosomes
Most human have 46 chromosomes in
their body cells
 Sperm cells and egg cells only have 23
chromosomes each
 Sperm and egg join to form a cell called
a zygote with 46 chromosomes total
 Together, the 46 human chromosomes
carry 50,000-100,000 genes


Basic steps to Meiosis:
› Chromosomes make a copy of themselves
› The cell divides into two new cells
› Each new cell then divides again
› RESULT: One parent cell results in four new
sex cells
› **Each sex cell contains half the number of
chromosomes of the parent cell
Humans have 46 chromosomes; they
consist of 23 pairs
 Each chromosome that makes up a pair
comes from a different parent
 For 22 of the pairs, the two chromosomes
look alike
 However, the chromosomes that make
up the 23rd pair look different from each
other
these are sex chromosomes


These two chromosomes determine a
person’s sex

There are two types of sex chromosomes
› X and Y
› Females have two X chromosomes
› Males have one X and one Y chromosomes

Parents pass one of their sex
chromosomes on to their offspring

A male can pass an X or a Y
chromosome to its offspring

A female can only pass an X
chromosome to its offspring

Human offspring have a 50% chance of
being male or female
Male
Parent
X
Y
X
X
XX
XX
(female)
(female)
XY
XY
(male)
(male)
Female parent
Offspring
50% chance of
being female (XX)
50% chance of
being male (XY)
Morgan used fruit flies rather than pea
plants to learn about chromosomes and
genes
 Fruit flies are easy to study because:

› Their cells have only 4 pairs of chromosomes
› The chromosomes are large; easy to see
› Fruit flies reproduce quickly
› It’s easy to tell the female fly from the male

Traits that are linked to the sex of an
organism are called sex-linked traits

Fruit flies usually have red eyes

Morgan noticed one male fly with white
eyes

He mated the white-eyed male with a redeyed female

This resulted in the F1 Generation and all
flies had red eyes (dominant trait)

Morgan found that when he mated flies
from his F1 Generation to produce an F2
Generation, some flies had white eyes
and some had red

However, all white-eyed flies were male

Morgan concluded that the white eye
color in fruit flies is linked to the sex of the
fly

A female fruit fly has two X
chromosomes

A male has one X and one Y
chromosome

Morgan found that the gene for eye
color in fruit flies is on the X chromosome

There is no gene for eye color on the Y
chromosome

This explains why eye color in fruit flies is a
sex-linked trait
Sex Chromosome
 Sex-linked Trait
 Carrier

Self-Check questions, pg. 245
 COMPLETE SENTETCES!!


Human genetics is the study of how
humans inherit traits

Humans have more chromosomes, do
not reproduce quickly, and cannot be
used in experiments

Scientists study heredity in humans by
studying identical twins

Identical Twins
› Sperm and egg join, form one zygote
› Zygote divides into two cells that separate
› Cells have identical genes

Fraternal Twins
› Form from two different zygotes
› Zygotes develop into offspring with different
sets of genes
Your genes determine your skin color,
eye color, body shape, and other
characteristics
 But your environment can also affect
your characteristics
 Scientists study identical twins who have
been separated since birth
 Both twins have identical genes, but
grew up in different environments


Food, sunlight, air and other parts of the
environment can affect characteristics

A person who doesn’t have good
nutrition may not grow tall, even though
they have the gene for tallness

X-rays and some types of chemicals
cause changes in genes, called
mutations
DNA in chromosomes is the material that
contains an organism’s genes
 DNA passes the genes from one cell to
another during cell division
 DNA is a large molecule shaped like a
twisted ladder
 The rungs of the ladder are made of four
different kinds of molecules called bases

The order of the bases in the DNA of a
cell provides a code for all the
information that the cell needs to live
 Different organisms have different orders
of bases
 The greater the difference between
organisms, the greater the difference in
order of bases
 The four bases are abbreviated as:

› T, A, C, and G

The order of bases in a frog’s DNA is very
different from the order of bases in human
DNA

But, the difference in the order of bases
between your DNA and your friend’s DNA is
not as great

The order of bases in a gene for hair color,
for example, determines whether the hair
will be black, red, brown, or blonde
DNA can replicate, or copy, itself
 DNA molecules are held together at the
rungs of the ladder
 The bases pair up in certain ways
 When DNA replicates, it first splits down
the middle of its rungs
 The paired bases separate, then new
bases pair with the separated bases


The result is two identical copies of the
original DNA molecule

DNA replication occurs every time a cell
divides normally

Pairing of Bases:
› Base A pairs with Base T
› Base C pairs with Base G

Sometimes there is a change in the order of
bases in a DNA molecule

Parts of the environment and chemicals
can cause mutations

Mutations cause changes in genes

Genes determine traits, so any mutation
can affect the traits of an organism
Mutations can be harmful, helpful, or
have no effect to the organism
 A genetic disease is a disease that results
from the genes a person inherits
 Recessive genes cause most genetic
diseases
 That means a person must inherit the
recessive gene from both parents in
order to have the disease

A gene pool is all the genes that are
found within the population
 The larger the population, the larger the
gene pool
 People living in a small population, often
mate others who may have the same or
similar genes
 Sexual reproduction among people in a
small gene pool is called inbreeding

Inbreeding can cause a disease called
hemophilia
 This disease causes the blood to not
have a certain protein that it needs to
clot or clump
 People who have hemophilia bleed
excessively when slightly injured
 Hemophilia was known as the “Royal
Disease” WHY??

A number of human traits are sex-linked
 The gene for color blindness in sex-linked
 Color blindness is more common in males
because the recessive gene is found on
the X chromosome
 Hemophilia and muscular dystrophy are
also sex-linked
 Both are found on the X chromosome, so
they are both more common in males

Mutation
 Base
 Genetic Disease

Replicate
Gene Pool
Inbreeding
Self-Check Questions, pg. 251
 COMPLETE SENTENCES!!


Farmers use breeding techniques to
grow hardier and better-tasting crops

Animal breeders use breeding
techniques to make prizewinning horses

These are examples of people affecting
the traits that organisms inherit

A mutation that results in an animal with
white fur would be helpful to that animal
in a snowy region

Over long periods of time, mutations
may lead to changes in population

This is one way that new species are
formed
Farmers and scientists use their
knowledge of genetics to produce new
varieties of plants and animals
 Selective breeding is selecting useful
mutations and breeding organisms so
the mutation shows up again
 Examples: Short legged sheep, cows that
produce large amounts of milk, pink
grapefruits and navel oranges

Selective breeding is used to produce
great race horses
 The breeder will select two parents with
desirable genes
 The will also select parents that have
good temperaments
 An animal’s genes also affect its
behavior

The process of introducing new genes
into an organism is genetic engineering
 Began in the 1970’s when scientists
transferred genes from one species of
bacteria to another species of bacteria
 Today, scientists transfer genes between
entirely different organisms
 Used to treat certain diseases or
conditions of plants and animals

Applied Genetics
 Selective Breeding
 Genetic Engineering

Self-Check questions, pg. 256
 COMPLETE SENTENCES!!
