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title. Genetics. Genetics. 22.1 What is genetics? 22.2 Chromosomes, genes and DNA 22.3 How are genes passed on from generation to generation? 22.4 Studying the pattern of inheritance 22.5 How to solve problems involving monohybrid inheritance? 22.6 Sex determination in humans

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Title

title

Genetics


Title

Genetics

22.1What is genetics?

22.2Chromosomes, genes and DNA

22.3How are genes passed on from generation to generation?

22.4Studying the pattern of inheritance

22.5 How to solve problems involving monohybrid inheritance?

22.6Sex determination in humans

22.7How to study human inheritance?

22.8Variation among individuals of the same species

22.9Why are we all different?

22.10 Significance of variations


22 1 what is genetics

22.1 What is genetics?

Why are you similar

to your parents in

many ways?

Passing on of characteristics from one generation to the next

is

the process is

the study is

heredity

inheritance

genetics


22 2 chromosomes genes and dna

22.2 Chromosomes, genes and DNA

chromosome

nucleus

chromatids

cell

DNA (helical)

gene


Title

Gene

  • short segment of aDNA at specific location of chromosome

  • basic unit of heredity

  • controls inheritable characteristics


Title

twisted together into a helix

deoxyribonucleic

acid

D

N

A

carries genetic information that determines the sequence of amino acids in proteins

types of proteins and enzymes control

  • body characteristics

  • metabolic activities


Title

e.g.human:46chromosomes;

cat: 38

chromosomes

DNA molecules

special proteins

Chromosome

specific

number of chromosomes in each species

same

numberin

all body cellsexcept sex cells


Title

Chromosomes

chromosomes in pairs (homologous chromosomes)

Body cells

diploid

either paternal or maternal chromosome

Sex cells (gametes)

haploid

Two haploid gametes combine to form one diploid cell


22 3 how are genes passed on from generation to generation

22.3 How are genes passed on from generation to generation?

starting point of life

male gamete

female gamete

zygote

haploid gametes

  • each carries half of the genes which determine the characteristics of the parent

  • as vehicles of inheritance


Title

Stages in meiotic cell division

1 diploid cell

4 haploid gametes

meiotic cell division

meiosis (nuclear division which reduces the chromosome number by half)

followed by division of cytoplasm

Stages in meiotic cell division


Title

Occurrence of meiotic cell division

flowering plants

  • male gametes in pollen grains

  • egg cells in ovules

animals

  • sperms in testes

  • ova in ovaries


Title

The significance of meiosis

  • Halving of chromosome numbers in gametes

  • produces haploid gametes

    • the diploid number of chromosomes can be restored at fertilization

  • Independent assortment

  • produces gametes with different genetic make-up

    • variations among offspring of the same species


Title

The two possible combinations of chromosomes in the gametes formed from only 2 pairs of chromosomes during meiosis

combination 1

either

Imagine the enormous

number of

combinations in

humans which have

23 pairs of

chromosomes!

combination 2

or


Title

Carry out Practical 22.1

Observation of meiosis in a testis squash of the grasshopper or in photomicrographs


22 4 studying the pattern of inheritance

22.4 Studying the pattern of inheritance

Monohybrid inheritance

  • inheritance of a characteristic which is controlled by only one pair of alleles for each gene

  • first studied by Gregor Mendel

who investigated the inheritance of two contrasting characters (tall & short stems) in garden peas


Title

About the experiment

dwarf

(pure breeding)

tall

(pure breeding)

produced offspring only

tall


Title

About the experiment

dwarf

(pure breeding)

tall

(pure breeding)

produced offspring only

dwarf


Title

About the experiment

dwarf

(pure breeding)

tall

(pure breeding)

cross-pollination & fertilization

first filial (F1) generation

all tall (1064)

self-pollination & fertilization of F1

second filial (F2) generation

787 tall

277 dwarf

3 : 1

???


Title

Other characters studied

Character studied

Shape of seed coat

Colour of cotyledons

Colour of seed coat

Shape of pods

inflated

smooth

yellow

grey

Cross

constricted

All show similar pattern of inheritance.

wrinkled

green

white

F1

all smooth

all yellow

all grey

all inflated

Ratio of characters in F2

2.96 : 1

3.01 : 1

3.15 : 1

2.95 : 1


Title

Interpretation of experimental results

  • These characteristics are controlled by pairs of genes.

tall

dwarf

parents

TT

tt


Title

Interpretation of experimental results

  • Each gamete receives only one gene (allele) from each pair.

tall

dwarf

parents

TT

tt

gametes

T

t


Title

Interpretation of experimental results

  • All offspring (F1) are tall because T gene is dominant to t gene.

tall

dwarf

parents

TT

tt

gametes

T

t

F1

Tt(tall)


Title

Interpretation of experimental results

  • F1 generation can produce 2 types of gametes (T or t).

tall

dwarf

parents

TT

tt

gametes

T

t

F1

Tt(tall)

gametes

T

t


Title

Interpretation of experimental results

  • There are 4 possible combinations of gametes when random fertilization happens.

tall

dwarf

parents

TT

tt

gametes

T

t

F1

Tt(tall)

Tt(tall)

gametes

T

t

T

t

F2

TT

Tt

Tt

tt

tall

dwarf

ratio

3 : 1


Title

Common terms in genetics

  • Allele

tall

dwarf

parents

  • a length of DNA controlling a certain character

TT

tt

which may have 2 or more alternate forms

gametes

T

t

F1

Tt(tall)

Tt(tall)

  • each form of a gene is called an allele

gametes

T

t

T

t

e.g. T : the allele for tallness

t : the allele for dwarfness

F2

TT

Tt

Tt

tt

tall

dwarf

ratio

3 : 1


Title

Common terms in genetics

  • Phenotype

tall

dwarf

parents

  • the appearance of a character of an organism

TT

tt

gametes

T

t

e.g. the phenotype of the F1 generation is tall

F1

Tt(tall)

Tt(tall)

gametes

T

t

T

t

F2

TT

Tt

Tt

tt

tall

dwarf

ratio

3 : 1


Title

Common terms in genetics

  • Genotype

tall

dwarf

parents

TT

tt

  • the genetic make-up of an organism in relation to the gene being investigated

gametes

T

t

F1

Tt(tall)

Tt(tall)

e.g. the genotype of the F1 generation is Tt

gametes

T

t

T

t

F2

TT

Tt

Tt

tt

tall

dwarf

ratio

3 : 1


Title

Common terms in genetics

  • Homozygote

tall

dwarf

parents

TT

tt

  • an organism whose genotype contains two identical alleles for a particular characteristic

gametes

T

t

F1

Tt (tall)

Tt (tall)

e.g. the tall parent plants (TT) or the dwarf plants (tt)

gametes

T

t

T

t

F2

TT

Tt

Tt

tt

homo= the same

tall

dwarf

ratio

3 : 1


Title

Common terms in genetics

  • Heterozygote

tall

dwarf

parents

TT

tt

  • an organism whose genotype contains two different alleles for a particular characteristic

gametes

T

t

F1

Tt (tall)

Tt (tall)

e.g. the tall plants with Tt make-up in F1 generation

gametes

T

t

T

t

F2

TT

Tt

Tt

tt

hetero = different

tall

dwarf

ratio

3 : 1


Title

Common terms in genetics

  • Dominant

tall

dwarf

parents

  • a dominant gene can express itself or produce its effect in both homozygous and heterozygous conditions

TT

tt

gametes

T

t

F1

Tt (tall)

Tt (tall)

e.g. T represents a dominant gene which causes the plants to be tall in either the homozygous (TT) or heterozygous (Tt) condition

gametes

T

t

T

t

F2

TT

Tt

Tt

tt

tall

dwarf

ratio

3 : 1


Title

Common terms in genetics

  • Recessive

tall

dwarf

parents

  • a recessive gene can only express itself in a homozygous condition

TT

tt

gametes

T

t

e.g. t represents a recessive gene which causes the plant to be dwarf when in a homozygous (tt) condition

F1

Tt (tall)

Tt (tall)

gametes

T

t

T

t

F2

TT

Tt

Tt

tt

tall

dwarf

ratio

3 : 1


Title

Common terms in genetics

  • Hybrid

tall

dwarf

parents

  • an individual which results from crossing two homozygous parents which are genetically different

TT

tt

gametes

T

t

F1

Tt (tall)

Tt (tall)

e.g. the F1 generation

gametes

T

t

T

t

F2

TT

Tt

Tt

tt

tall

dwarf

ratio

3 : 1


Title

Examples of monohybrid inheritance

Alleles

Phenotype

Dominant

Recessive

Characteristic

  • In maize plants Colour of the seeds in a cob

dark-coloured e.g. purple

light-coloured e.g.yellow

  • In fruit flies Length of wingAppearance of abdomen

long

vestigial

broad

narrow

  • In mice Coat colourEar size

black

brown

normal

short


Title

Examples of monohybrid inheritance

Dominant characteristics

Recessive characteristics

brown eye

blue eye

  • Eye colour

lobed ear

lobeless ear

  • Ear lobes

tongue can be rolled

tongue cannot be rolled

  • Tongue rolling

normal pigment

  • Skin pigment

no pigment (albinism)

straight thumb

curved thumb

  • Thumb shape

Examples of human characteristics that are inherited in a Mendelian manner


22 5 how to solve problems involving monohybrid inheritance

22.5 How to solve problems involving monohybrid inheritance?

all can be solved by genetic diagrams and Punnett squares

We are often provided with the phenotypes(P) and genotypes(G) of the parents, and then asked to:

  • deduce P and/or G of the offspring and their P and/or G ratios

  • determine the probability that an offspring will have a certain P or G

  • predict the number of offspring that will have a certain P or G


Title

Genetic diagrams

Consider a monohybrid cross between two humans which are heterozygous for the presence of ear lobes.

Let

L = allele for lobed ear (dominant)

l = allele for lobeless ear (recessive)

  • Choose a symbol to represent the alleles of the character of the parents.


Title

Genetic diagrams

Let

L = allele for lobed ear (dominant)

  • a State the genotypes

  • and phenotypes of

  • the parents.

l = allele for lobeless ear (recessive)

Lobed ear

Lobed ear

Ll

Ll

Parents

b Label the genotypes

‘Parents’.

c Write ‘’ between

the phenotypes and

genotypes of the

parents to stand for

a cross.

always write dominant allele first


Title

Genetic diagrams

Let

L = allele for lobed ear (dominant)

  • a Write down the

  • possible alleles of

  • the male and female

  • gametes.

l = allele for lobeless ear (recessive)

Lobed ear

Lobed ear

Ll

Ll

Parents

Label them ‘Gametes’.

Gametes

L

l

L

l

b Add lines as

shown.


Title

Genetic diagrams

Let

L = allele for lobed ear (dominant)

  • a Show the results of

  • possible random

  • crossing of the

  • gametes by using

  • lines as shown.

l = allele for lobeless ear (recessive)

Lobed ear

Lobed ear

Ll

Ll

Parents

Label the offspring ‘F1’ generation.

Gametes

L

l

L

l

b State the

phenotypes of the

offspring

underneath the

genotypes.

LL

Ll

Ll

ll

F1

Lobed ear

Lobeless ear


Title

Punnett squares

  • Draw four boxes as shown.


Title

Punnett squares

  • Write down the possible alleles from the female along the top, one above each box.

Also do the same along the left side for the male, one next to each box.

(female)

and

(male).

Label them

L

l

L

l


Title

Punnett squares

  • The products of the various possible combinations after fusions are written in the appropriate boxes.

L

l

LL

Ll

L

Ll

ll

l


Title

Punnett squares

  • State the phenotypes and genotypes of the offspring.

L

l

LL

Ll

L

lobed ear

lobed ear

Ll

ll

l

lobed ear

lobeless ear


Title

The outcome of any particular cross is totally unrelated to that of any other.

It is possible to predict the proportion of offspring that will have a certain phenotype or genotype.

The use of genetic diagrams or Punnett squares only gives the expected results.

from heterozygous couples, 3/4 and 1/4 of offspring will have lobed ears and lobless ears respectively

You should note that:


Title

Offspring

first parent  second parent

phenotypic ratio

genotypic ratio

1

BB

BB

2

BB

Bb

3

Bb

Bb

4

BB

bb

5

Bb

bb

6

bb

bb

Types of monohybrid crosses

  • up to 6 types for any pair of alleles

e.g. eye colour in human (B:dominant allele for brown eyes, b: recessive allele for blue eyes)

all brown eyes

all BB

all brown eyes

BB : Bb = 1 : 1

brown : blue = 3 : 1

BB : Bb: bb = 1 : 2 : 1

all Bb

all brown eyes

Bb : bb = 1 : 1

brown : blue = 1 : 1

all blue eyes

all bb


Title

Types of monohybrid crosses

If you are familiar with the 6 types of crosses, you should be able to:

  • Find the phenotypes or genotypes of parents if result of certain crosses is provided.

  • State the dominant or recessive characters for a certain phenotype if the phenotypes of the parents and offspring are provided.


Title

How to find out the genotype of an organism with a particular dominant phenotype

a dominant phenotype

2 possible genotypes

homozygous dominant

heterozygous dominant

How can we find out the genotype accurately ?

By test cross.


Title

How to find out the genotype of an organism with a particular dominant phenotype

Example: To identify the genotype of a tall plant

Case 1

If the genotype is TT

organism to be tested

homozygous recessive organism

tt

TT

Gametes

T

t

Tt

F1 genotype phenotype

tall

(all the offspring are tall)


Title

How to find out the genotype of an organism with a particular dominant phenotype

Example: To identify the genotype of a tall plant

Case

2

If the genotype is

Tt

organism to be tested

homozygous recessive organism

tt

Tt

Gametes

T

t

t

tt

Tt

F1 genotype phenotype

tall

dwarf

1 : 1

ratio


Title

organism to be tested

homozygous recessive organism

organism to be tested

homozygous recessive organism

tt

tt

Tt

TT

Gametes

Gametes

T

t

t

T

t

tt

Tt

F1 genotype phenotype

Tt

F1 genotype phenotype

tall

dwarf

tall

1 : 1

(all the offspring are tall)

ratio

How to find out the genotype of an organism with a particular dominant phenotype

Example: To identify the genotype of a tall plant

if both tall and dwarf offspring are obtained

if all offspring are tall

unknown organism

= homozygous dominant

unknown organism

= heterozygous dominant


Title

Carry out Practical 22.2

Observation of maize cobs with grains of different colours


22 6 sex determination in humans

22.6Sex determination in humans

the 23rd pair of chromosomes

X

X

X

Y

X

X

X

Y

  • sex is determined by the sex chromosomes

Cell of a male

Cell of a female

has one X chromosome and one Y chromosome

has two X chromosomes

all eggs carry

one X chromosome

50% sperms carry X and 50% carry Y

XY

XY

XY become boys

XX become girls

XX

XX


22 7 how to study human inheritance

22.7How to study human inheritance?

male

female

individual

generation

male

female

1

2

brown eye

blue eye

3

4

5

6

7

8

9

10

11

12

13

14

15

16

  • by studying pedigree or family tree

and tracing the pattern of inheritance of some easily recognizable characters

An example of a pedigree showing the inheritance of eye colour


22 8 variation among individuals of the same species

22.8Variation among individuals of the same species

Have you ever met

two people who

are exactly alike ?

The differences in characteristics among individuals of the same species

variations

discontinuous variations

continuous variations


Title

There is a continuous range of intermediate phenotypes between extremes of a quality.

The characters are controlled by many genes and may be affected by environment.

  • Continuous variation

e.g. weight, height


Title

1200

1100

1000

900

800

700

number of people in each height

600

500

400

300

200

100

140

145

150

155

160

165

170

175

180

185

190

height (cm)

  • Continuous variation

  • a normal distribution curve can be obtained


Title

There are no intermediates.

No normal distribution curve can be produced.

The characters may be controlled by one pair of genes and are less easily affected by environment.

  • Discontinuous variation

e.g. tongue rolling, ear lobes


Title

Carry out Practical 22.3

Observation of variations in humans, e.g. height variation, tongue rolling


22 9 why are we all different

22.9Why are we all different?

  • variation is a result of

Heredity

Environment

or

interacts with


Title

Heredity

caused by heredity

Genetic variation

is a result of

independent assortment of chromosomes at meiosis

random fertilization

mutation

  • a sudden, relatively permanent inheritable change in the DNA of a gene or more than one gene

  • only the one occurs in gamete cells can be passed to the next generation


Title

Environment

results in

environment can affect the expression of certain genes

variation

an egg fertilized by a sperm

zygote divides into two cells

each cell continues to divide


Title

Environment

results in

environment can affect the expression of certain genes

variation

two balls of cells develop into two genetically identical individuals


Title

Environment

results in

environment can affect the expression of certain genes

variation

brought up in a well-nourished environment

brought up in a poorly-nourished environment

identical twins


Title

Environment

Other examples

  • light

e.g. the gene for chlorophyll production

chlorophyll produced

no chlorophyll produced


Title

parents

nn

nn

gametes

n

n

nn

fertilized eggs

develop at 25°C

develop at 16°C

F1

nn

nn

Environment

Other examples

2temperature

e.g. the gene for curly wings in fruit flies


22 10 significance of variations

22.10Significance of variations

Variations exist in the length of necks in a population of giraffes, some with long necks, some with short necks.

When food supply is enough, neck length is not a determining factor in the survival of the giraffes.


22 10 significance of variations1

22.10Significance of variations

When the weather becomes dry for a long time, no grass grows on the ground.

Short-necked giraffes will die.

Long neck length becomes a favourable variation.

It allows giraffes to get leaves on tree tops, hence to survive.


22 10 significance of variations2

22.10Significance of variations

Long-necked giraffes survive and pass on this characteristic to their offspring.


22 10 significance of variations3

22.10Significance of variations

Variations cause some individuals to be better adapted to the environment than others.


Title

is the study of

occurs by action of

explains

may show

Concept diagram

Genetics

inheritance

characteristics

genes

continuous variation

discontinuous variation


Title

are made up of segments of

can be

are located on

occur as pairs of

and some proteins make up

character expressed in both

character expressed only in

with different numbers in

determine the

Concept diagram

genes

DNA

dominant

recessive

chromosomes

alleles

heterozygous condition

homozygous condition


Title

represents each cell’s

represents an organism’s

formed by

formed by

Concept diagram

chromosomes

alleles

with different numbers in

determine the

diploid cells

haploid cells

genotype

phenotype

mitotic cell division

meiotic cell division

genetic make-up

observable features

(in relation to the gene studied)

(in relation to the character studied)


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