Presentation on theme: "Look at the characters below: 1) Who are their parents? What could they be called? 2) Why do they look like their parents?"— Presentation transcript:
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 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 3/10 Complete worksheets
MitosisMeiosis 2 daughter cells 4 daughter cells Genetically identical to parents Genetically not Identical to parents diploidhaploid 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 in Growth - where in animals? where in plants? Repair – what? Replacement – cells which wear away or die e.g.? Asexual reproduction – e.g. All over the body In 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 XXXY XY
Genetic Crosses Punnet Square Who determines the gender of the baby, Mum or Dad? Parental phenotypeFemale x Male Parental genotype XX x XY GametesX X x X Y Outcome: 50% Girls + 50% Boys GenotypeXX PhenotypeGirl GenotypeXY PhenotypeBoy Gametes XY XXXXY XXXXY
Boy or girl - what decides? All egg cells contain an X chromosome. XXXXY XY 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 genes Chromosomes 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 genes Every 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. MF
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 FFff The genes must have the same letter but the dominant gene is always in capitals Symbols 4
Chromosomes and genes Every 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 MF 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 phenotype If BB is homozygous dominant Bb and bb are Bb
Chromosomes and genes If you resemble your mum and/or your dad, it’s because your cells contain a mixture of their genes. MF 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 genes Suppose 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. 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 genes 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? There are 2 23 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 genes Remember 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’.
Patterns of inheritance Sarah 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 inheritance Whether 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 inheritance We 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. RRrRrr 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 inheritance SarahTim 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 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 inheritance SarahTim 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 inheritance When 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 Rrrr rr
Patterns of inheritance At fertilisation, random chance decides whether the sperm (with one r allele) meets an egg with an R allele, or one with an r allele. Rr r Rrrr 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. Rr 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 The 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