3b genetics & inheritance

Question 1

definition of a gamete?

Answer 1

sex cells

Question 2

definition of a chromosome?

Answer 2

thread like structures of DNA, carrying genetic information in the form of genes. located in nucleus of the cell

Question 3

definition of a gene?

Answer 3

a section of DNA found on chromosomes that codes for a specific protein/a trait

Question 4

definition of an allele?

Answer 4

different versions/variations of a particular gene

Question 5

definition of a dominant allele?

Answer 5

an allele that’s always expressed, even if only one copy is present

Question 6

definition of a recessive allele?

Answer 6

an allele that’s only expressed if two copies are present (therefore no dominant allele present)

Question 7

definition of homozygous?

Answer 7

if the two alleles of a gene are the same

Question 8

definition of heterozygous?

Answer 8

if the two alleles of a gene are different

Question 9

definition of a genotype?

Answer 9

the combination of alleles that control each characteristic

Question 10

definition of a phenotype?

Answer 10

the observable characteristics of an organism, or the genotype that is expressed

Question 11

true or false: some characteristics are controlled by a single gene

Answer 11

true, the inheritance of these single genes is monohybrid inheritance

Question 12

what’s an example of monohybrid inheritance?

Answer 12

fur colour in mice, red-green colour blindness in humans

Question 13

do we have two alleles for each gene, and why?

Answer 13

• yes
• 2 copies of each chromosome, 2 copies of each gene, therefore 2 alleles for each gene
• 1 of the alleles inherited from mother, other from father

Question 14

a dominant allele needs to be inherited from ___ parent in order for the characteristic to show up in the phenotype

Answer 14

a dominant allele needs to be inherited from one parent in order for the characteristic to show up in the phenotype

Question 15

a recessive allele needs to be inherited from ____ parents in order for the characteristic to show up in the phenotype

Answer 15

a recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype

Question 16

what do homozygous dominant and homozygous recessive mean?

Answer 16

homozygous dominant: having two copies of the dominant allele

homozygous recessive: having two copies of the recessive allele

Question 17

what letters are given to dominant and recessive alleles when completing genetic diagrams?

Answer 17

dominant: capital letter
recessive: same letter but lower case

Question 18

what does polygenic mean?

Answer 18

characteristics that are controlled by more than one gene

(most characteristics are a result of multiple genes interacting, rather than a single gene)

Question 19

what can the phenotypes of polygenic characteristic show?

Answer 19

a wide range of combinations in features

Question 20

what does polygenic inheritance mean?

Answer 20

the inheritance of polygenic characteristics

Question 21

why do we not usually use genetic diagrams for polygenic inheritance?

Answer 21

because it’s difficult to show due to the wide range of combinations

e.g. eye colour: while it’s true brown eyes dominant to blue eyes, not as simple as this as eye colour’s controlled by several genes. this means there’s several different phenotypes beyond brown & blue, like green & hazel

Question 22

how can the inheritance of characteristics be determined?

Answer 22

using Punnet squares

Question 23

what do punnet squares show?

Answer 23

the possible combinations of alleles that could be produced in the offspring

Question 24

what do F1 and F2 mean?

Answer 24

first generation and second generation

Question 25

what are family tree diagrams/pedigrees usually used for?

Answer 25

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 probability that some1 in family will inherit the genetic disorder

Question 26

3.21B what is codominance?

Answer 26

- when two alleles cross that are both dominant - when this happens the organism may show multiple phenotypes

Question 27

how do you represent codominance on a punnet square?

Answer 27

use same capital letter for each allele & add another letter to top right-hand corner to represent their differences —> both alleles are dominant, therefore both need 2 be represented by capital letter

Question 28

3.14 what is a genome?

Answer 28

the entire DNA of an organism

Question 29

3.14 what is a gene?

Answer 29

a section of a molecule of DNA that codes for a specific protein

Question 30

3.16B describe a DNA molecule

Answer 30

two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G)

Question 31

what are nucleotides

Answer 31

nucleotides consist of a sugar (deoxyribose), a phosphate & a nitrogenous base (nitrogenous base are A, C, G or T)

Question 32

3.16B what does adenine always pair with?

Answer 32

thymine

Question 33

3.16B what does cytosine always pair with?

Answer 33

guanine

Question 34

what type of bond joins the nucleotides together?

Answer 34

covalent bonds between the sugar of one nucleotide & the phosphate group of the next nucleotide

Question 35

3.17B how does RNA differ from DNA?

Answer 35

- RNA single stranded, DNA double stranded - RNA has sugar ribose while DNA has sugar deoxyribose - RNA has base uracil (U) while DNA has base thymine (T)

Question 36

3.18B what is a codon?

Answer 36

the sequence of three bases in DNA that codes for a specific amino acid

Question 37

3.18B what is messenger RNA/mRNA?

Answer 37

- formed in **nucleus** of cell - copies sequence of bases of a section of DNA in **transcription** - carries code for building a specific protein from nucleus to ribosomes in cytoplasm (acts as messenger)

Question 38

3.18B what is tRNA?

Answer 38

- found in **cytoplasm** - picks up specific amino acids from cytoplasm & brings them into position on surface of ribosome where they can be joined together in specific order to make a specific protein (**translation**)

Question 39

3.18B describe transcription

Answer 39

1. 2 strands of DNA helix **unzipped** by breaking weak hydrogen bonds between base pairs. this unwinding of the helix caused by enzyme **helicase** 2. enzyme **RNA polymerase** attaches to DNA just before the gene 3. RNA polymerase moves along DNA strand. free RNA nucleotides are **attracted to** (and form hydrogen bonds w) the exposed DNA strand nucleotides by complementary base pairing 4. RNA polymerase then **catalyses** formation of **covalent bonds** between RNA nucleotides to form strand of **mRNA** --> bc opposite base bonds w exposed DNA bases, strand of mRNA is opposite copy of DNA strand - **complementary copy** 5. newly formed strand of mRNA now ready to **leave nucleus & travel to ribosome**

Question 40

3.18B describe translation

Answer 40

1. the **mRNA** strand travels through cytoplasm & attaches to **ribosome**, passes through ribosome 2. for every 3 mRNA bases the ribosome lines up 1 complementary molecule of tRNA. (every 3 bases on mRNA is a **codon**, complementary 3 bases on **tRNA** is an **anticodon**) --> bc there are 3 mRNA bases for each tRNA molecule, we call this the **triplet code** 3. tRNA molecules transport **specific amino acids** to ribosome. ribosome allows 2 tRNA molecules to sit next to each other; ribosome **catalyses** formation on a **covalent bond** (peptide bond) between 2 amino acids 4. used tRNA molecules **exit ribosome** & **collect another specific amino acid** 5. chain of several hundred amino acids formed in correct order according to og DNA then made - called a **polypeptide** --> after translation, polypeptide finally folded into correct shape & becomes **protein**

Question 41

3.22 what type of inheritance are most phenotypic features the result of?

Answer 41

polygenic inheritance, rather than single genes

Question 42

3.34 what is mutation?

Answer 42

a rare, random change in genetic material that can be inherited (or from topic notes: a change in the sequence of bases in DNA)

Question 43

3.35B how can a change in DNA affect the phenotype of an organism?

Answer 43

by **altering the sequence of amino acids in a protein**: - replacement of a single base pair w a different pair - addition or deletion of one or more base pairs this will normally alter the shape & function of the protein

Question 44

3.36B what effect do most genetic mutations have on the phenotype?

Answer 44

most genetic mutations have no effect on the phenotype, some have a small effect and rarely do they have a significant effect

Question 45

3.37B how can the incidence of mutations can be increased?

Answer 45

by exposure to ionising radiation: - gamma rays - x-rays - ultraviolet rays and some chemical mutagens: - e.g. chemicals in tobacco

Question 46

3.38 explain Darwin’s theory of evolution by natural selection

Answer 46

1. **variation** occurs due to **mutations**. these occur randomly & continuously 2. some individuals within pop. will have features (specialisations) which make them more able to survive in the environment 3. these individuals w better specialisations more likely to **survive** to **reproduce** 4. they'll **pass on** their **advantageous alleles** to next generation 5. therefore the offspring will tend to inherit advantageous characteristics 6. less favourable alleles tend to become less common in pop 7. repeats for many generations 8. over time, specialisations which help organisms survive will become more common & characteristics of species will change

Question 47

3.39 understand how resistance to antibiotics can increase in bacterial populations, and appreciate how such an increase can lead to infections being difficult to control

Answer 47

1. originally, none of bacteria resistant & penicillin not used 2. doctors began to use penicillin, which killed bacteria 3. a chance **mutation** gave some bacteria **resistance** to the antibiotic 4. these bacteria **better adapted** to an environment in which penicillin used 5. they survived in greater no.s. as there are fewer non-resistant bacteria there is less competition for resources 6. so resistant bacteria **reproduce rapidly** & pass on **resistance allele** 7. this **repeats** w each generation of bacteria 8. proportion of bacteria w resistance allele inc until most had the allele

Question 48

3.28 what does mitosis produce?

Answer 48

- genetically identical daughter cells - these cells are diploid - 1 pair of each chromosome in each nucleus

Question 49

3.29 what is mitosis used for/when does it occur?

Answer 49

growth, repair, cloning and asexual reproduction

Question 50

3.28 what are the stages of mitosis?

Answer 50

1. the cell grows 2. the **DNA replicates** (is copied) to form 2 copies of each chromosome. these copies (**chromatids**) are joined together at a single point (the centromere) 3. nuclear membrane breaks down; the **chromosomes line up** across the centre of the cell, attached to special spindle fibres 4. the **chromatids** (copies of chromosomes) are **pulled apart** & move to opposite ends of the cell 5. the **cytoplasm and cell membrane divide** to form 2 identical cells

Question 51

3.30 what does meiosis produce?

Answer 51

- four genetically different haploid cells - they each have half the number of chromosomes (haploid) - the cells formed are gametes

Question 52

3.30 what are the stages of meiosis?

Answer 52

1. the parent cell grows 2. the **DNA replicates** (is copied) to form 2 copies of each chromosome. these copies (**chromatids**) are joined together at a single point (the centromere) 3. the nuclear membrane breaks down; the **homologous pairs of chromosomes join together** 4. the homologous pairs of chromosomes **exchange sections of DNA**. they then attach to the spindle fibres in pairs, at the centre of the cell 5. the **homologous pairs of chromosomes** are **pulled apart** & move to opposite ends of the cell. the cell divides 6. the chromosomes line up along the centre of the cell, attached to the spindle fibres. the **chromatids separate** & move to opposite ends of the cell. each cell divides again

Question 53

3.31 how is variation in offspring produced?

Answer 53

- by genetic variation in gamete cells produced by meiosis - random fertilisation of ova (egg cells) by male gametes

Question 54

3.32 what are the haploid and diploid number of chromosomes in humans?

Answer 54

haploid: 23 diploid: 46

Question 55

3.33 what are the different types of variation that a species can have?

Answer 55

genetic, environmental, or a combination of both

Question 56

what are differences in the roles of mitosis and meiosis?

Answer 56

mitosis: - generates all adult cells except gametes (growth & repair) - occurs throughout human body - occurs throughout a plant - used for asexual reproduction (cloning) meiosis: - only used to produce gametes - only occurs in ovary and testis - only occurs in ovary and anther - used for sexual reproduction

Question 57

what are differences in the processes of mitosis and meiosis?

Answer 57

mitosis: - produces genetically identical daughter cells - produces two cells - involves one division - produces diploid cells in humans - keeps same chromosome number meiosis: - produces genetically non-identical daughter cells - produces four cells - involves two cell divisions - produces haploid cells in humans - halves number of chromosomes

Question 58

3.15 where are genes located?

Answer 58

the nucleus of a cell contains chromosomes; genes are located on these