inheritance

Question 1

Inheritance is

Answer 1

the transmission of genetic information from generation to generation

Question 2

Chromosomes are located in

Answer 2

the nucleus of cells

Question 3

chromosomes are

Answer 3

thread-like structures of DNA, carrying genetic information in the form of genes

Question 4

A gene is

Answer 4

a short length of DNA found on a chromosome that codes for a specific protein

Question 5

Genes control

Answer 5

our characteristics as they code for proteins that play important roles in our cells

Question 6

Alleles are

Answer 6

different versions of the same gene; for example, the gene for eye colour has alleles for different colours like brown and blue:

Question 7

Diploid & Haploid Nuclei

Answer 7

All humans have 23 different chromosomes in each cell

In most body cells, not including the gametes (sex cells), we have 2 copies of each chromosome, leading to a total of 46 chromosomes

Nuclei with two sets of chromosomes are known as diploid nuclei

The gametes (egg and sperm) only have one copy of each chromosome, meaning they have a total of 23 chromosomes in each cell

Nuclei with one set of unpaired chromosomes are known as haploid nuclei

Question 8

Sex, or gender, is determined by

Answer 8

an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a few pairs of genes)

Question 9

Females have the sex chromosomes

Answer 9

XX

Question 10

Males have the sex chromosomes

Answer 10

XY

Question 11

As only the father can… they are responsible for

Answer 11

• pass on either an X or Y chromosome,
• pass on either an X or Y chromosome,

Question 12

XX & XY Chromosomes shown using s genetic diagram

Answer 12

Question 13

the sequence of bases in a gene is

Answer 13

the genetic code for putting together amino acids in the correct order to make a specific protein

Question 14

Explain how a protein is made

Answer 14

– the gene coding for the protein remains in the nucleus – mRNA molecules carry a copy of the gene to the cytoplasm – the mRNA passes through ribosomes – the ribosome assembles amino acids into protein molecules – the specific order of amino acids is determined by the sequence of bases in the mRNA

Question 15

DNA controls cell function by

Answer 15

controlling the production of proteins (some of which are enzymes), antibodies and receptors for neurotransmitters

Question 16

Although all body cells in an organism contain the same genes,

Answer 16

many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs

Question 17

Mitosis

Answer 17

Question 18

Stem Cells

Answer 18

Many tissues in the human body contain a small number of unspecialised cells

These are called stem cells and their function is to divide by mitosis and produce new daughter cells that can become specialised within the tissue and be used for different functions

Question 19

mitosis is used for

Answer 19

growth, repair of damaged tissues, replacement of cells and asexual reproduction

Question 20

Mitosis is defined as

Answer 20

nuclear division giving rise to genetically identical cells

Question 21

cells divide their chromosomes double beforehand in mitosis because

Answer 21

it ensures that when the cell splits in two, each new cell still has two copies of each chromosome (is still diploid)

Question 23

Meiosis is a

Answer 23

a type of nuclear division that gives rise to cells that are genetically different

Question 24

meiosis is used to

Answer 24

produce the gametes (sex cells)

Question 25

Meiosis

Answer 25

The number of chromosomes must be halved when the gametes (sex cells) are formed Otherwise there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg) This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete We describe gametes as being haploid – having half the normal number of chromosomes Because of this double division, meiosis produces four haploid cells eiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made This means that when gametes fuse randomly at fertilisation, each offspring will be different from any others

Question 26

Differences between Mitosis & Meiosis

Answer 26

Question 27

A gene is

Answer 27

a short length of DNA found on a chromosome that codes for a particular characteristic (expressed by the formation of different proteins)

Question 28

Alleles are

Answer 28

variations of the same gene

Question 29

phenotype

Answer 29

The observable characteristics of an organism

Question 30

genotype

Answer 30

The combination of alleles that control each characteristic

Question 31

Alleles can be dominat or recessive

Answer 31

A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype If there is only one recessive allele, it will remain hidden and the dominant characteristic will show If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same) An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele) If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different) When completing genetic diagrams, alleles are abbreviated to single letters The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case

Question 32

homozygous

Answer 32

If the two alleles of a gene are the same

Question 33

heterozygous

Answer 33

f the two alleles of a gene are different

Question 34

When completing genetic diagrams, alleles are

Answer 34

abbreviated to single letters The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case

Question 35

In the example above, the phenotypes and genotypes from left to right would be:

Answer 35

Phenotype – brown eyes; genotype – BB (homozygous dominant) Phenotype – blue eyes; genotype – bb (homozygous recessive) Phenotype – brown eyes; genotype – Bb (heterozygous)

Question 37

we cannot always tell the genotype of an individual for a particular characteristic just by looking at the

Answer 37

phenotype

Question 38

If two individuals who are both identically homozygous for a particular characteristic are bred together

Answer 38

they will produce offspring with exactly the same genotype and phenotype as the parents – we describe them as being ‘pure breeding’ as they will always produce offspring with the same characteristic

Question 39

A heterozygous individual can pass on different alleles for the same characteristic each time it is bred with any other individual and can therefore

Answer 39

produce offspring with a different genotype and phenotype than the parents – as such, heterozygous individuals are not pure breeding

Question 40

Monohybrid inheritance is

Answer 40

the inheritance of characteristics controlled by a single gene (mono = one)

Question 41

Monohybrid inheritance an be determined using a

Answer 41

genetic diagram known as a Punnett square

Question 42

A Punnett square diagram shows

Answer 42

s the possible combinations of alleles that could be produced in the offspring From this the ratio of these combinations can be worked out Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case You should always write the dominant allele first, followed by the recessive allele

Question 43

The height of pea plants is controlled by a single gene that has two alleles: tall and short The tall allele is dominant and is shown as T The small allele is recessive and is shown as t ‘Show the possible allele combinations of the offspring produced when a pure breeding short plant is bred with a pure breeding tall plant’

Answer 43

The term ‘pure breeding’ indicates that the individual is homozygous for that characteristic The tall plant’s genotype is TT The short plant’s genotype is tt The possible combination of alleles in the offspring, shown in a Punnett square, is:

Question 44

‘Show the possible allele combinations of the offspring produced when two of the offspring from the first cross are bred together’

Answer 44

All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are:

Question 45

‘Show the results of crossing a heterozygous plant with a short plant’ The height of pea plants is controlled by a single gene that has two alleles: tall and short The tall allele is dominant and is shown as T The small allele is recessive and is shown as t

Answer 45

The heterozygous plant will be tall with the genotype Tt The short plant is showing the recessive phenotype and so must be homozygous recessive – tt The results of this cross are as follows:

Question 46

How to construct Punnett squares

Answer 46

Determine the parental genotypes Select a letter that has a clearly different lower case, for example: Aa, Bb, Dd Split the alleles for each parent and add them to the Punnett square around the outside Fill in the middle four squares of the Punnett square to work out the possible genetic combinations in the offspring You may be asked to comment on the ratio of different allele combinations in the offspring, calculate a percentage chances of offspring showing a specific characteristic or just determine the phenotypes of the offspring Completing a Punnett square allows you to predict the probability of different outcomes from monohybrid crosses

Question 47

Identifying an Unknown Genotype

Answer 47

Breeders can use a test cross to find out the genotype of an organism showing the dominant phenotype This involves crossing the unknown individual with an individual showing the recessive phenotype – if the individual is showing the recessive phenotype, then its genotype must be homozygous recessive By looking at the ratio of phenotypes in the offspring, we can tell whether the unknown individual is homozygous dominant or heterozygous

Question 48

‘A plant breeder has a tall plant of unknown genotype. How can they find out whether it is homozygous dominant or heterozygous?’

Answer 48

The short plant is showing the recessive phenotype and so must be homozygous recessive – tt If the tall plant is homozygous dominant, all offspring produced will be tall:

Question 49

Family Pedigrees

Answer 49

Family pedigree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family This can be used to work out the probability that someone in the family will inherit the genetic disorder Males are indicated by the square shape and females are represented by circles Affected individuals are red and unaffected are blue Horizontal lines between males and females show that they have produced children (which are shown underneath each couple) The family pedigree above shows: both males and females are affected every generation has affected individuals That there is one family group that has no affected parents or children the other two families have one affected parent and affected children as well

Question 50

Codominance

Answer 50

Some genes have alleles that are equally dominant and so are both expressed equally in the phenotype This is known as codominance Both codominant alleles are shown with upper case letters in genetic diagrams, but the letters used are different

Question 51

For example, feather colour in hens may be white, black or speckled (it has both white feathers and black feathers) The alleles can be shown as... There are three possible genotypes:... There are also three possible phenotypes:

Answer 51

- W for white and B for black - WW, BB and BW - WW = white, BB = black, and BW = speckled

Question 52

Inheritance of Blood Group

Answer 52

Inheritance of blood group is an example of codominance There are three alleles of the gene governing this instead of the usual two Alleles IA and IB are codominant, but both are dominant to IO I represents the gene and the superscript A, B and O represent the alleles IA results in the production of antigen A in the blood IB results in the production of antigen B in the blood IO results in no antigens being produced in the blood These three possible alleles can give us the following genotypes and phenotypes: image is wrong

Question 53

‘Show how a parent with blood group A and a parent with blood group B can produce offspring with blood group O’

Answer 53

The parent with blood group A has the genotype IAIO. The parent with the blood group B has the genotype IBIO. We know these are their genotypes (as opposed to both being homozygous) as they are able to produce a child with blood group O and so the child must have inherited an allele for group O from each parent Parents with these blood types have a 25% chance of producing a child with blood type O

Question 54

Sex-Linked Characteristics

Answer 54

When alleles that control a particular characteristic are found on the sex chromosomes, we describe the inheritance that results as ‘sex linked’ In almost all cases, there are only alleles on the X chromosome as the Y chromosome is much smaller Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia) Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring If that offspring is a male, he will have the disease The results of a cross between a normal male and a female who is a carrier for colourblindness is as follows:

Question 55

Sex-Linked Characteristics ad diseases

Answer 55

Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia) Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring If that offspring is a male, he will have the disease The results of a cross between a normal male and a female who is a carrier for colourblindness is as follows: