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INHERITANCE. What is Inheritance?. G R Davidson. What is Inheritance?. Inherited characteristics are determined by genetic information received from parents. This information is passed on in the genes on the chromosomes.

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inheritance

INHERITANCE

What is Inheritance?

G R Davidson

what is inheritance
What is Inheritance?
  • Inherited characteristics are determined by genetic information received from parents.
  • This information is passed on in the genes on the chromosomes.
  • Each organism receives 2 pieces of genetic information about each characteristic, one from each parent.
  • These may be different or the same.
slide3

Tall

x

Tall

Dwarf

x

e.g. Pea plants can either be tall or dwarf.

Tall

Tall

Tall

Dwarf

Tall

Dwarf

Tall

Dwarf

phenotypes
Phenotypes
  • For each inherited characteristic an organism possesses there may be a number of different forms.
  • These forms are called PHENOTYPES.
  • The phenotype describes the appearance of the inherited characteristic, e.g. flower colour may be red, white, etc..
phenotypes5

= male

= ability to roll tongue

= female

= inability to roll tongue

Granny

Grandad

Grandad

Granny

Aunt

Dad

Mum

Uncle

Brother

Sister

Brother

Sister

Phenotypes

Since each organism receives 2 pieces of information for this characteristic, we now need to work out the GENOTYPE.

genotypes
Genotypes

RR

rr

rr

rr

rr

rr

Rr

Rr

rr

rr

Rr

Rr

The GENOTYPE is the set of genes the organism possesses.

genotypes7
Genotypes
  • The way in which inherited characteristics are passed on from parents to offspring, follows a pattern.
  • Genetics is the study of these patterns.
  • If the two genes in a pair are identical, we say they are TRUE BREEDING or HOMOZYGOUS e.g. TT or tt
  • If they are different, we say they are HETEROZYGOUS e.g. Tt
alleles
Alleles
  • Different forms of the same gene are called ALLELES.
  • E.g. tall and dwarf are different alleles of the same gene.
chromosomes
Chromosomes
  • Chromosomes are thread-like structures found in the nucleus of a cell, which carry inherited information.
  • Each normal body cell has 2 matching sets of these chromosomes.
  • In humans there are 23 pairs of chromosomes.
gamete production
Gamete Production
  • Chromosomes are passed on to the next generation in the sex cells (gametes).
  • Each sex cell will contain only 1 of each pair of chromosomes – therefore will only have 23 chromosomes.
  • This allows it to join up with (fertilise) another gamete and make the numbers back up to 23 pairs.
genetic cross
Genetic Cross
  • When we study a characteristic, the first thing we do is assign it a letter e.g. black would be B or tall T.
  • Each characteristic has 2 forms,e.g. tall and dwarf.
  • One of these forms will be DOMINANT over the other, which we say is RECESSIVE.
  • We assign the dominant trait the capital letter and the recessive trait the small letter. E.g. tall is dominant to dwarf, therefore tall will be T and dwarf will be t.
genetic cross12
Genetic Cross
  • This means a plant with the genotype TT will have the phenotype TALL
  • The plant with the genotype Tt will have the phenotype TALL
  • The plant with the genotype tt will have the phenotype dwarf.
ratios of phenotypes
Ratios of Phenotypes
  • The simplest genetic crosses only involve one characteristic and these are called MONOHYBRID CROSSES.
  • The patterns of inheritance can be followed in a Punnett Square.
  • E.g.
punnett square

Gametes for tall parent

Gametes for dwarf parent

Punnett Square

T = tall and t = dwarf

If we cross a homozygous tall with a dwarf plant the following punnett square would be produced.

In this cross ALL the offspring are Tt and are therefore, tall.

punnett square15

T

t

T

t

Punnett Square

If both plants are heterozygous, what would the square look like?

Gametes

TT

Tt

Tt

tt

sex chromosomes

X

X

X

Y

Sex Chromosomes

Sex in humans is determined by the sex chromosomes. Females have 2 X chromosomes (XX) and males have an X and a Y chromosomes (XY).

Gametes

XX

XX

XY

XY

And so, half of the offspring will be female and the other half male.

monohybrid crosses

B

B

b

b

Monohybrid Crosses

We always set out a cross the same way:

Parents (P) Black x Albino

BB

bb

Gametes

B or B

b or b

The gametes for one parent go along the top.

Bb

Bb

The gametes for the other parent go down the side.

Bb

Bb

We can then complete the square.

The results of this cross are that all the F1 are black.

monohybrid crosses18

B

b

B

b

Monohybrid Crosses

If we now cross 2 of the F1 generation:

Parents (P) Black x Black

Bb

Bb

Gametes

B or b

B or b

The gametes for one parent go along the top.

BB

Bb

The gametes for the other parent go down the side.

Bb

bb

We can then complete the square.

The results of this cross are that 3 of the F1 are black and 1 is albino.

monohybrid crosses19

B

b

b

b

Monohybrid Crosses

Now try one of your own:

Parents (P) Black x Albino

Bb

bb

Gametes

B or b

b or b

The gametes for one parent go along the top.

Bb

bb

The gametes for the other parent go down the side.

Bb

bb

We can then complete the square.

The results of this cross are that half of the F1 are black and half are albino.

monohybrid crosses20
Monohybrid Crosses
  • Your answer should always have the parental genotypes.
  • Your answer should always have the parental gametes.
  • Your answer should always have a completed punnett square.
  • Your answer should always have the F1 phenotypes and the ratio they occur in.