UNIT 17 Inheritance

Question 1

chromosomes are made of…

Answer 1

made of DNA,
which contains genetic information in the form of genes

Question 2

Define a gene

Answer 2

a length of DNA that codes for a
protein

Question 3

allele

Answer 3

an alternative form of a gene

• everyone has two alleles for each gene, one on each chromosome

Question 4

inheritance

Answer 4

transmission of genetic information from generation to generation

Question 5

desc: inheritance of sex in humans with
reference to X and Y chromosomes

Question 6

the sequence of bases in a gene
determines…

Answer 6

the sequence of bases in a gene determines the sequence of amino acids used to make a specific protein

amino acid sequence determines folded protein shape

Question 7

what does DNA do

Answer 7

DNA controls cell function by
controlling the production of proteins,

including enzymes, membrane carriers and receptors for neurotransmitters

Question 8

Explain how a protein is made

Answer 8

• DNA remains in the nucleus
• MESSENGER RNA (mRNA) is a copy of a gene
• mRNA molecules are made in the nucleus and move to the cytoplasm
• the mRNA binds with ribosomes
• the ribosome assembles amino acids into protein molecules [by ribosomes convert genetic seq. into a chain of amino acids which fold to form a protein]
• the specific sequence of amino acids is determined by the sequence of bases in the mRNA

Question 9

“central dogma” - dna to rna to protein

dna => rna, transcription
rna => dna, reverse transcription

rna => protein, translation

Answer 9

[NOT NECCESSARY]

Question 10

DNA

Answer 10

• double helix

Question 11

explain why many genes in a particular cell are not expressed

Answer 11

most body cells in an organism contain the same genes, BUT many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs

Question 12

haploid nucleus

e.g. sperm, egg, pollen

Answer 12

a nucleus
containing a single set of chromosomes

Question 13

diploid nucleus

Answer 13

a nucleus containing
two sets of chromosomes

Question 14

in a diploid cell…

Answer 14

in a diploid cell, there is a pair of each type of chromosome

and in a human diploid cell there are 23 pairs

Question 15

a pair of chromosomes = homologous chromosomes,

which means

Answer 15

SAME chromosomes, similar structure & shape

Question 16

mitosis -

Answer 16

nuclear division giving rise to
genetically identical cells [diploid, identical to parent cell]

• exact replication of chromosomes
  occurs BEFORE mitosis, then cell divides [mitosis]

Question 17

role of mitosis -

type of cell division resulting in production of new plants = mitosis

Answer 17

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

Question 18

during mitosis…

Answer 18

during mitosis,

📌 the copies of chromosomes separate, maintaining the chromosome number in each daughter cell

[half the chromosomes will move into each divided cell, creating 2 daughter cells with the same chromosome number as the original parent cell]

Question 19

stem cells

Answer 19

unspecialised cells that
divide by mitosis to produce daughter cells that can become specialised for specific functions

Question 20

meiosis

Answer 20

a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells

• is involved in the production
  of gametes/sex cells.
  only occurs in testes in males & ovaries in females

Question 21

what does meiosis produce

Answer 21

• produces 4 haploid daughter cells with half the chromosome number of a regular body cell
• bc at fertilisation, the 2 haploid gametes will fuse, making the zygote diploid

Question 22

genotype

Answer 22

the genetic make-up of an
organism and in terms of the alleles present

Question 23

phenotype

Answer 23

the observable features of
an organism

(physical appearance of organism)

Question 24

homozygous?

pure-breeding?

Answer 24

having two identical
alleles of a particular gene

BB or bb

• two identical homozygous individuals that breed together will be pure-breeding

Question 25

heterozygous

HETERO = DIFFERENT

Answer 25

having two different
alleles of a particular gene

Bb

• a heterozygous individual will not be pure-breeding

Question 26

dominant allele

Answer 26

Represented by B (capital letter)

📌 the allele that is
expressed if it is present [in the genotype]

Question 27

recessive allele

Answer 27

rep. by (lower case letter) b

📌 an allele that is only expressed when there is no dominant allele of the gene present in the genotype

Question 28

Interpret pedigree diagrams for the inheritance of a given characteristic [usually disease]

diagram rules

Answer 28

• Males are indicated by the square shape and females are represented by circles
• Affected individuals are red and unaffected are blue

Question 29

Explain how to use a TEST CROSS [!!!] to identify an unknown genotype - of an organism showing the dominant phenotype

Answer 29

📌 crossing the unknown individual with an individual showing the recessive phenotype

📌 RESULT: if the individual is showing the recessive phenotype, then its genotype must be heterozygous. NOT showing recessive phenotype, must be homozygous dominant

Question 30

how to tell whether the unknown individual is homozygous dominant or heterozygous

Answer 30

By looking at the ratio of phenotypes in the offspring

Question 31

fit to question - use Punnett Squares

e.g.

Answer 31

If the tall plant is homozygous dominant, all offspring produced will be tall

If the tall plant is heterozygous, half the offspring will be tall and the other half will be short

Question 32

monohybrid inheritance

Answer 32

inheritance of characteristics controlled by a single gene

Question 33

codominance

Answer 33

a situation in which both alleles in heterozygous organisms contribute to the phenotype

Question 34

linked trait/gene

Answer 34

linked traits are FOUND ON THE SAME CHROMOSOME

This means they tend to be inherited together more frequently than by random chance.

Question 35

a sex-linked characteristic

Answer 35

📌 a feature in which the gene responsible is located on a sex chromosome

📌 and that this makes the characteristic more common in one sex than in the other

• almost always, only alleles on X chromosome as Y chromosome is much smaller

Question 36

why males show sex linkage more & are much more likely to express a recessive phenotype for sex linked traits

Answer 36

📌 males only have one X chromosome; they’re much more likely to show sex-linked recessive conditions

If males inherit a recessive allele on their X chromosome it will always be expressed as there is no other allele to mask it

📌 Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele [becoming a carrier]

; have twice the chance of inheriting a dominant allele

Question 37

e.g. of sex linkage

Answer 37

red-green colour blindness

Question 38

multiple allele traits

Answer 38

Involves genes that have more than two different alleles

Question 39

e.g of codominance - explain inheritance of ABO blood groups:

phenotypes are A, B, AB and O blood groups

and alleles are…

Answer 39

• alleles = Iᴬ [ results in the production of antigen A in the blood], Iᴮ [ production of antigen B in the blood], and Iᴼ [no antigens being produced in the blood]
• Alleles Iᴬ and Iᴮ are codominant, but both are dominant to Iᴼ

Question 40

inheritance table - genotype, phenotype

Answer 40

IᴬIᴬ or IᴬIᴼ => A

IᴮIᴮ or IᴮIᴼ => B

IᴬIᴮ => AB

IᴼIᴼ => O

Question 41

farmers cut runners & sell new plants.
suggest advantages of producing new strawberry plants this way [2]

Answer 41

any 2 from:

• quicker to produce new plants
• same characteristics in offspring as parent plant / identical / clones produced
• cheap

Question 42

desc how a section of DNA determines structure of a protein [4]

Answer 42

• gene (coding for protein) remains in the nucleus
• sequence of bases of mRNA determines sequence of amino acids
• 3 bases for one amino acid
• several amino acids make up a protein
  [mRNA made, mRNA binded to ribosome]

Question 43

A flower grower bred red carnations.
Describe how growers selectively breed plants.

Answer 43

any 3 from:

📌 cross (parent) plants with desired feature ;

📌 (grow seeds and) chose offspring for (desired) feature(s) ;

📌 cross (offspring) plants showing features with each other ;

[many generations of, crossing and selecting ;]

Question 44

Explain the disadvantages of using sexual reproduction to breed red carnations.

Answer 44

any 2 from:

📌 two parents / gametes, are required ;

📌 variation in offspring/ offspring might not all be red ;

📌 time consuming ;

📌 limited number of seeds

Question 45

Describe the phenotype of a heterozygous carnation plant for this gene.

Answer 45

pink (flowers)

[red & white combined]

Question 46

Explain why coat colour is an example of discontinuous variation. [3]

just as w/ blood groups

Answer 46

📌distinct, phenotypes / categories ;

📌 controlled by genes ;

📌not affected by the, environment

Question 47

Explain why meiosis occurs at P and mitosis occurs at Q. [3]

meiosis at P

mitosis at Q

Answer 47

📌P meiosis

reduction division/ CHROMOSOME NUMBER IS HAVLED ;

prevents doubling of chromosome number at fertilisation ;

HAPLOID GAMETES ; + gamete production ;

📌Q mitosis

GROWTH IS TAKING PLACE ;
PRODUCING GENETICALLY IDENTICAL CELLS ;

Question 48

State where DNA is located in a cell. [2]

Answer 48

in chromosomes ;
in the nucleus ;

Question 49

suggest why there is very little genetic variation in offspring of adult nematode

Answer 49

any two of:

• self-fertilisation
• gametes from same individual
• only new source of variation is mutation

Question 50

Sickle cell anaemia is not found throughout the whole world. Most cases of the disease
occur in sub-Saharan Africa and in parts of Asia. The distribution is similar to that for the
infectious disease malaria.

Explain why the distribution of sickle cell anaemia and malaria are similar.[5]

Answer 50

malaria, is severe disease/may be fatal;

HbNHbN more likely to die (of malaria) before have children (to pass
on genes);

HbN HbS / sickle cell carriers, do not die from sickle cell anaemia;

HbN HbS / sickle cell carriers, have children (and pass on genes);

and pass on the (Hbs) allele;

Question 51

Explain how discontinuous variation differs from continuous variation, in its expression and
cause. [3]

Answer 51

discontinuous variation – influenced by genes alone;

discontinuous variation – no effect of the environment

[discontinuous variation, is discrete]

limited number of phenotypes;

Question 52

A sample of blood was taken from a person with sickle cell anaemia and examined with
an electron microscope.
Fig. 4.1 shows some of the red blood cells in the sample.

Explain the problems that may occur as these cells circulate in the blood system. [4]

+ Describe the harmful effects on the body of having red blood cells which become sickle-shaped. [5]

Answer 52

(red blood cells) get stuck in capillaries

reduce , supply of, oxygen / nutrients ( to tissues / cells / muscles) ;

reduce , removal of, carbon dioxide / wastes, (from tissues / cells /
muscles) ;

pain ;

increased chance of blood clotting

+ “fewer red blood cells. less haemoglobin. haemoglobin / blood, less efficient at transporting oxygen. less respiration. less energy / fatigue. pain. ‘sickle cell crisis’.”

Question 53

Explain how the results of the crosses show that these alleles for flower colour are not dominant or recessive.

Answer 53

codominance
both alleles are expressed
the phenotype is different from either parent and the phenotype is a mixture of the two colours/intermediate

Question 54

very small number of flour beetles have
white eyes.
(d) Explain how this happens and why they are so rare

Answer 54

mutation ;
mutation, rare event ;
(white) allele is recessive ;
only expressed in homozygote recessive ;