Look at the characters below:

1) Who are their parents? What could they be called?

2) Why do they look like their parents?

Inheritance 27/9

Chromosomes, genes and DNA

Objectives:*Define inheritance

** State the meaning of the terms chromosome , gene and DNA

Starter:What is inheritance?

Inheritance is transfer of characteristics or traits e.g. physical appearance, personality, capabilities at things like sports, music etc from one generation to the next.

• Chromosome • Gene – • DNA –• Haploid cell • Diploid cell During cell division the chromosomes appear very tightly coiled at other

times they are uncoiled so that cells can use the information coded in the DNA

animation

Each chromosome is a length made of deoxyribonucleic acid wound around molecules of protein and very tightly coiled. Each gene (a section of DNA)is an unit of inheritance in that it

codes for a specific protein. Humans have about 20-25000 different genes A long chain molecule made of double helix of 4 bases

is one with a single set of chromosomes (for example a gamete)

-is one with a double set of chromosomes (for example any somatic cell )

Mitosis and Meiosis 30/9

Objectives:*Define mitosis and state its importance in growth, development and asexual reproduction** Define meiosis and state its importance in the formation of gametes and in producing genetic variation

animation

H/w due in 3/10Complete worksheets

MITOSIS

MEIOSIS

Mitosis Meiosis

2 daughter cells 4 daughter cells

Genetically identical to parents Genetically not Identical to parents

diploid haploid

In somatic cells for growth, repair, replacement

In sex cell producing organs e.g.

For asexual reproduction For sexual reproduction

Mitos don’t do sex

Importance of mitosis inGrowth - where in animals? where in plants?

Repair – what? Replacement – cells which wear away or die e.g.?Asexual reproduction – e.g.

All over the bodyIn root, shoot tips

wounds

RBC

Fungus, amoeba, bacteria, some plants

Genes and Chromosomes 3/10

Objectives:*Describe the inheritance of sex based upon X and Y chromosomes in humans**Use the terminology of inheritance ***Distinguish between genes and alleles

H/w due in on 4/10 Write the definitions of the following words:Genotype, phenotype, homozygous, heterozygous, dominant, recessive

Boy or girl - what decides?The 23rd pair of chromosomes in females is a matching pair called the X chromosomes.

Males have only one X chromosome: its ‘partner’ is a much smaller Y chromosome.

Boy or girl - what decides?The photograph below shows all the

chromosomes in a woman’s cells. There are 23 pairs.

The photograph below shows all the chromosomes in a man’s cells.

How do the male and female sets of chromosomes differ?

Boy or girl - what decides?When a cell in a woman’s ovaries divides to

make egg cells, each egg receives one X chromosome.

When a cell in a man’s testes divides to make sperms, each sperm receives either an

X or a Y chromosome.

XX

X X

XY

X Y

Genetic Crosses Punnet SquareWho determines the gender of the baby, Mum or Dad?

Parental phenotype Female x Male

Parental genotype XX x XY

Gametes X X x X Y

Outcome: 50% Girls + 50% Boys

Genotype XX

Phenotype Girl

Genotype XY

Phenotype Boy

Gametes X Y

X XX XY

X XX XY

Boy or girl - what decides?

All egg cells contain an X chromosome.

XXX XY

X Y

50% of sperms contain an X chromosome, the other 50% a Y.

If this sperm fertilises the egg, the baby is a

girl.

If this sperm fertilises the egg, the baby is a boy.

Since there is an equal chance of an X sperm or a Y sperm fertilising the egg, boys and girls are born in roughly equal numbers.

Chromosomes and genesChromosomes resemble tiny threads in the nucleus of a cell.

Each chromosome is a string of units called genes: each gene is an ‘instruction’ which controls or affects a particular feature of the body.

Chromosomes and genesEvery chromosome you received from your mother at fertilisation has a matching ‘partner’ from your father.

The partner chromosomes may have identical copies of some genes, but slightly different versions of others.

If you resemble your mum and/or your dad, it’s because your cells contain a mixture of their genes.

M F

The genes are represented by letters

The gene for black fur is given the letter F

The gene for brown fur is given the letter f

FF ff

The genes must have the same letter but thedominant gene is always in capitals

Symbols 4

Chromosomes and genesEvery chromosome you received from your mother at fertilisation has a matching ‘partner’ from your father.

The partner chromosomes may have identical copies of some genes, but slightly different versions of others.

These different or identical versions are called alleles. When the alleles are same they are homozygous when different heterozygous

M F

Here, mother’s

and father’s

chromo-somes

have identical versions of these genes...

... but different versions of these.

Lets take the first pair of genes and name them as B now since the two alleles are different lets call them B,b. In such cases sometimes one is dominant over the other we write the dominant one with capital letters and the other as small letter and call it recessive. If we assume that these genes represent eye colour and B= brown and b = blue when B is present we can get two genotypes BB or Bb and their phenotypes will all be brown but we can also get an allele combination of bb which will give a blue phenotypeIf BB is homozygous dominantBb and bb are

Bb

Chromosomes and genesIf you resemble your mum and/or your dad, it’s because your cells contain a mixture of their genes.

M F

But if you have brothers or sisters, they inherited a mixture of your mum’s and dad’s genes

too: so why aren’t you exactly like them?

Chromosomes and genesSuppose that these are two of the 23 pairs of chromosomes in a woman’s body cells.

When she makes egg cells, each egg must receive only one chromosome from each pair. (Why is this important?)

Any egg could receive...

... these two

or these two

or these two

or these two.

For just two pairs of chromosomes, there are four different ways of choosing one from each pair.

How many different ways are there of choosing one from each of 23 pairs?

Chromosomes and genesFor just two pairs of chromosomes, there are four different ways of choosing one from each pair.

How many different ways are there of choosing one from each of 23 pairs?

There are 223 different ways: that’s over 8 million.

So if you have a sister, there’s a one-in-8-million chance that the egg that grew into you contained the same set of chromosomes as the egg that grew into your sister; and another one-in-8-million chance that the sperm that fertilised your egg contained the same set of chromosomes as the sperm that fertilised your sister’s.

The chance of you and your sister getting the same set of chromosomes from both parents is therefore one in 64 million million.

Chromosomes and genesRemember that the chromosomes are strings of genes - coded instructions that control many of your characteristics.

So if you have a sister, about half (on average) of the genes you inherited from your parents will be the same as hers; and half will be different. The chance of you both inheriting exactly the same set of genes is so tiny that in practice it never happens.

In your exercise books, write

•a paragraph headed ‘Why brothers and sisters look similar’

•a paragraph headed ‘Why brothers and sisters do not look identical’.

Monohybrid inheritance 4/10

Objectives:*Define monohybrid inheritance**Explain examples of monohybrid crosses

Patterns of inheritanceSarah can roll her tongue into a tube, like this:

So can her baby brother Tim:

But her twin brother Matt can’t do it:

Sarah’s mum can roll her tongue too, but her dad can’t. What decides who can and who can’t?

Patterns of inheritanceWhether or not you can roll your tongue depends on which versions of a particular gene you possess.

R This version, R, gives you the ability to roll your tongue.

r This version, r, does not give you the ability to roll your tongue.

Patterns of inheritanceWe all possess two copies of the tongue-rolling gene, on one of our pairs of chromosomes.

If both your copies are the R version, you can roll your

tongue.

R R rR r r

If both your copies are the r version,

you cannot roll your tongue.

If you have one R and one r, you can roll your tongue.

Patterns of inheritance

rR

If you have one R and one r, you can roll your tongue.

The ‘versions’ R and r are called alleles of this gene. If you have just one R allele and the other is r, you can roll your tongue just as well as if you had two R alleles. Because of this, the R allele is described as dominant, and r recessive.

Patterns of inheritanceSarah Tim Matt

Remember, their mum can roll her tongue, but their dad can’t.

What alleles of the tongue-rolling gene must Matt possess?

What alleles of the gene must their dad possess?

rr

rr

How did Matt get his two r alleles? One from each parent.

So, what alleles of the tongue-rolling gene must their mum possess?

Patterns of inheritanceSarah Tim Matt

If their mum can roll her tongue, but passed on an r allele to Matt, she must have...

Rr

So what decided who inherited tongue-rolling and who did not?

Patterns of inheritanceWhen cells in Sarah’s mum’s ovaries divided to make egg cells, each egg received either

an R or an r allele.

When cells in her dad’s testes divided to make sperms, each sperm could only receive

an r allele.

Rr

R r

rr

r r

Patterns of inheritanceAt fertilisation, random chance decides whether the sperm (with one r allele) meets an egg with an R allele, or one with an r allele.

R r

r Rr rr

If Sarah’s mum and dad have another child, what is the probability that he or she will be a tongue-roller?

50%: this is the probability that the egg will contain an R allele.

Patterns of inheritance• Mary and Jim can both roll their tongues• Their first two children, Laura and Jason, can also

roll their tongues• Their third child, Charlotte, cannot roll her tongue

Explain the inheritance of tongue-rolling in Mary and Jim’s family. What is the probability that their next child will be able to roll her/his tongue?

Patterns of inheritance• Mary and Jim can both roll their tongues, so must

both have an R allele.• But they have a child who can’t roll her tongue:

Charlotte must have inherited r from both her parents.

• So Mary and Jim must both be Rr.

At fertilisation, random chance decides which sperm meets which egg.

R r

R

r

Half of Mary’s eggs have R, half r.

Half of Jim’s sperms have R, half r.

Three times out of four, fertilisation will result in RR or Rr. Only one in four fertilisations will give rr.

?RR ?

??

Rr

Rr rrThe probability that their next child will be a tongue-roller is 3/4, or 0.75, or 75%.

Co-dominance 7/10

Objectives:*Explain the term codominance**Describe the inheritance of ABO blood groups