Unit 4 Biology – Continuity & Change
Chapter 7 – Heredity
Heredity –the study of inheritance (the ability to pass characteristics to successive generations)
Genetics –study of patterns & mechanisms of inheritance, through coded chemical messages passed from
one generation to the next.
Genes- sections of DNA that contain the code for the production of proteins. It is believed that genes may
interact with other genes to produce a protein or may code for more than 1 protein.
Genome – all of the DNA in a cell (differs in each species). Study of the genome is genomics.
Bio-informatics- managing and analysing biological data using advanced computing techniques.
Human genome has been mapped, function of each gene is now being investigated.
Chromosomes & heredity
Cells pass on instructions for growth & development to new cells during nuclear division –mitosis and
cytoplasmic division cytokinesis. These instructions are found in DNA, usually in the form of
chromosomes (within the nucleus of eukaryotes) that are specific for each species.
Asexual reproduction - offspring identical to parent, only 1 set of chromosomes is involved.
Sexual reproduction - variation in offspring, get chromosomes from 2 sources when gametes combine
In eukaryotes DNA exists as long fibres called chromatin. During division they become chromosomes –
visible double strands (sister chromatids) coiled around histone proteins, linked at the centromere and
existing as pairs (one of each pair coming from each parent).
Genes – regions on DNA that code for different proteins that determine characteristics or traits.
- Locus (loci) -location of a particular gene on a chromosome. Homologous chromosomes have
corresponding genes at the same locus.
- Alleles- alternative forms of the same gene (e.g. pea colour –alleles are yellow and green)
- Genotype –total set of genetic instructions in an organism
- Phenotype – physical, biochemical or physiological traits expressed by a person’s genotype
Karyotype – a display of the paired chromosomes from a cell, showing number, size and shape.
-Used to determine gender and chromosomal abnormalities (e.g extra chromosomes –Down’s
syndrome (47, +21), missing chromosomes, parts of chromosomes missing or duplicated)
-Each species has own number of chromosomes -> characteristic karyotype
-Somatic cells or normal body cells have 2 sets of chromosomes (diploid 2n). 2n =46 in humans
-Gametes contain 1 set or half the normal number of chromosomes (haploid 1n) 1n =23-humans
-Autosomes –chromosomes not responsible for sex determination exist in 22 homologous or
matching pairs, one of each pair having come from each parent
-Sex chromosomes determine gender XX= female, XY(non matched or heterosome)=male
Pg 206 - Review Questions 1, 2, 3, 4, 5
Biozone – 189/90 Chromosomes (all-understand structure, terminology), 191->95-Karyoptypes (all), 245
Human Genome project(all-understand mapped genes), 188-genomes (Q1), 247-genome projects (read)
Cell Division
In somatic cells nucleus divides first (mitosis) then cytoplasm (cytokinesis) -> produces 2 identical
diploid daughter cells.
In sex cells (gonads), a different type of cell division (meiosis) occurs, resulting in 4 different haploid
daughter cells.
Interphase – stage between nuclear divisions-> cell grows, replicates DNA & prepares for next division
-Cell volume and amount of DNA vary during different stages of cell cycle
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DNA replication
• Occurs during the S phase of the cell cycle
1. Enzyme DNA helicase “unzips” the DNA strand (chromatin) a section at a time, exposing
nucleotide bases (H bonds are separated) Junction b/w unwound & intact strand –replication fork
2. Free nucleotides within nucleus attach to exposed bases (corresponding pairs T-A, C-G), enzyme
DNA polymerase assists. A wrong base in the sequence (mutation) changes the code.
3. Enzyme DNA ligase seals new stretches of nucleotides into continuous strand that recoils.
4. 2 double helix DNA molecules produced, 1 parent strand, 1 new strand-called the semiconservative replication model
5. Chromosomes become visible – 2 chromatids (original and copy) joined at centromere
Mitosis
Produces 2 identical cells, used for growth, replacement of dead, damaged tissue, asexual reproduction
Stages• Prophase –chromatin threads condense, sister chromatids appear joined at centromere. Spindle
forms, nucleolus disappears & nuclear membrane breaks down.
• Metaphase – chromosomes move to middle of cell, line up on equator
• Anaphase – chromatids separate, move to opposite poles (each chromatid now a chromosome)
• Telophase – chromosomes lengthen, less visible, nuclear membrane forms, spindle disassembles.
Cytokinesis occurs dividing cytoplasm (cleavage) resulting in 2 new cells. Cell plate forms in plant cells.
Apoptosis - programmed series of events that lead to cell death, internal contents are dismantled by
enzymes called capsases. Uncontrolled cell division causes cancerous tissue (tumours). Cancer cells avoid
apoptosis. - Biobox 7.4 page 211
Division in prokaryotes
Prokaryotes have no nuclei, DNA forms a circular chromosome free in cytoplasm.
DNA strand is replicated & separated, a wall forms to divide cell in 2 –Binary fission
Other small DNA rings – plasmids may be present and reproduce independently.
Meiosis – division in sex cells
• Occurs in gonads (testes, ovaries)
• Produces haploid (1n -half chromosome number) gametes (ova, sperm)
• Prevents doubling up of chromosomes at fertilisation
There are 2 divisions of the nucleus, and 4 main stages in each division
• Prophase 1–chromatin threads condense, sister chromatids appear joined at centromere. Spindle
forms, nucleolus disappears & homologous chromosomes pair up or synapse.
o Crossing over of chromosome sections occurs. This produces new combinations of genes
on chromosomes, which increases variation in the population.
• Metaphase 1 – nuclear membrane breaks down, homologous chromosomes move together to
middle of cell, line up on equator
• Anaphase 1 – homologous chromosomes separate & move to opposite poles. This disjunction
occurs randomly & results in half the double stranded chromosomes at each end of the cell. (1n)
• Telophase 1-cell starts to split, nuclear membrane reforms, spindle breaks down (2 cells now)
• Prophase 2 – new spindle forms
• Metaphase 2 – chromosomes move to equator of spindle
• Anaphase 2 – chromatids separate, move to opposite poles
• Telophase 2 – spindle disappears, nuclear membrane reforms, chromosomes become threadlike
• Cytokinesis divides cytoplasm (4 cells now)
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Sex determination (in humans)
Eggs - 22 autosomes, 1 X chromosome; Sperm - 22 autosomes X or Y chromosome. (½ - X, ½ - Y)
Males end up with X from mum, Y from dad (XY); Females end up with X from mum, X from dad(XX)
Asexual reproduction produces little variation but uses less energy for complex reproductive processes.
Sexual reproduction produces variation between generations, making it more likely for some individuals
with slight variations to survive changing environmental conditions.
Pg 216 - Review Questions 8, 9, 10, 11
Biozone – 201/2 –Replication (q1,2,3), 203-division (q1,3), 204-mitosis(all), 287-meiosis(all - important
to understand this, crossing over, non-disjunction, 266->269 (fate of conceptions - read, aneuploidy q1-7,
Down’s q2, 5) 289/0 –cross over, 205-apoptosis (all), 298 – sex determination
Patterns of Inheritance
Two sets of homologous chromosomes, 1 from each parent = 2 copies of each gene (eg. flower colour).
There are different forms of the gene called alleles (eg. purple flower/white flower). The 2 alleles could be
the same (homozygous) or different (heterozygous).
Monohybrid Inheritance cross looking at 1 pair of traits
Gregor Mendel discovered principles of heredity by studying distinct characteristics of pea plants. He
cross-pollinated pure breeding plants and observed 1 trait at a time in offspring. Each pair of traits
showed one trait dominant (only a single allele required to show it – appears in heterozygous condition),
the other recessive (only appears if 2 alleles for it are present –is masked in heterozygous condition).
Example of a cross
Parental generation
-Pure tall plant x pure short plant
F1 (1st filial) Generation
-all tall plants – 2 of this generation crossed to produce F2
F2 Generation
-3 tall plants: short plant
Other traits tested showed the same set of results
1 trait is dominant (trait shown in F1 - tall) the other is recessive (masked in F1, reappeared in F2 -short)
Gene transmission
2 alleles for height are tall -T, short –t, dominant trait in capital, recessive in lower case.
Homozygous means pure breeding (2 alleles the same)
genotypes (genetic make up) TT and tt;
phenotypes (physical expression or appearance caused by genes) tall; short.
Heterozygous-2 different alleles for the trait -> genotype – Tt; phenotype – tall (dominant trait shows).
Environmental effects - Pg 218 Biobox
Environmental factors can affect appearance in genetically identical organisms. Eg. heat prevents pigment
formation in extremities of some rabbits & cats. Weight & height can be affected by nutrition. Chlorophyll
only develops if light is available, flowers appear if day length is suitable.
Biozone 295-gene/environment interaction (1, 4, 6)
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Predicting outcomes – Punnett squares
If genotypes are known we can determine what gametes are produced, and how they could combine.
Parents- Cross pure tall x pure short
Gametes produced
F1 - Possible combinations
F1 cross –
Gametes produced
F2 possible combinations
Gametes
½T
½t
Genotypes:
Phenotypes:
Genotypes TT and tt
all T
all t
only Tt (heterozygous tall plants)
Tt
x Tt
½ T, ½ t;
½T
¼ TT
¼ Tt
½ T, ½ t
½t
¼ Tt
¼ tt
¼ TT; ½ Tt; ¼ tt
¾ Tall; ¼ short -> ratio 3 tall:1 short.
Probability in genetic ratios
Ratios are only a prediction of probable numbers in large samples, or the probability of an outcome. Eg. a
plant has the probability of ¾ that it will be tall, ¼ chance of being short. Random nature of gamete
combination results in each outcome being independent of previous ones.
Test crosses
To find out the genotype of an organism with a dominant trait (tall - TT or Tt) it is crossed with an
organism with the pure recessive trait (short), then genotype can be determined. If only the dominant trait
appears in the F1 it is likely the tall parent was homozygous (TT), but if the recessive trait appears in some
offspring the tall parent must be heterozygous (Tt).
Pg 223 - Review Questions 12, 13, 14, 15, 16, 17, 18
Biozone – 273 Mendel’s peas (read), 274-Mendel’s laws (read), 275-Alleles (all), 276/7-crosses (all),
279/80-dominance (all), 281/2-traits
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Dihybrid inheritance – Inheritance of 2 pairs of characteristics
Parents - Tall plants with purple flowers x short plants with white flowers
F1 generation – all tall, purple -> shows tall & purple are dominant (used for F1 x F1 cross)
T – tall, t – short, P – purple, p - white
F3 generation – 9 tall purple:3 tall, white:3 short purple:1short, white
Expected ratios 9:3:3:1- dominant/dominant :dominant/recessive: recessive/dominant: recessive/recessive
Punnett Square shows possible genotypes, can be used to predict phenotypes
We assume 4 different types of gametes are produced in equal ratios. All combinations are assumed to be
equally possible. Independent assortment – assumes each allele for one gene can combine independently
with alleles for the other gene.
Gametes
¼ TP
¼ Tp
¼ tP
¼ tp
F2 phenotypes
¼ TP
1
/16 TTPP
tall, purple
1
/16 TTPp
tall, purple
1
/16 TtPP
tall, purple
1
/16 TtPp
tall, purple
9
/16 tall purple
¼ Tp
1
/16 TTPp
tall, purple
1
/16 TTpp
tall, white
1
/16 TtPp
tall, purple
1
/16 Ttpp
tall, white
3
/16 tall white
¼ tP
1
/16 TtPP
tall purple
1
/16 TtPp
tall, purple
1
/16 ttPP
short, purple
1
/16 ttPp
short, purple
3
/16 short purple
¼ tp
1
/16 TtPp
tall, purple
1
/16 Ttpp
tall, white
1
/16 ttPp
short, purple
1
/16 ttpp
short, white
1
/16 short white
Test crosses & explanations
Probability of being tall is ¾ , probability of being purple is ¾, of both is ¾ x ¾ = 9/16
Probability of being short is ¼ , probability of being white is ¼, of both is ¼ x ¼= 1/16
Can determine probability of 2 traits by multiplying each trait’s individual probability together.
Independent assortment occurs when the genes for the different traits being looked at are found on
different chromosomes. These chromosomes behave independently during meiosis allowing different
combinations to occur in different gametes.
Pg 228 - Review Questions 19, 20,
Biozone – 283/4-dihybrid,
Relationships & dominance –incomplete dominance & codominance
Gene expression can follow patterns other than dominant/recessive inheritance.
Incomplete/ partial dominance occurs when neither trait is dominant and the heterozygous phenotype is
a blend of the two homozygous phenotypes (eg. red, pink, white). Both alleles are represented by the same
capital letter with different superscripts: CR – red, CW – white. 2 white alleles CW CW result in a white
flower, 2 red alleles CR CR results in a red flower and 1 red 1white CR CW results in pink.
Codominance is very similar to incomplete dominance and it occurs when both traits are dominant and the
heterozygous phenotype shows both homozygous phenotypes (cattle -red, roan, white). Alleles represented
the same as incomplete dominance.
Table pg. 229 shows comparison of 3 types of dominant inheritance.
Multiple alleles-where there are more than 2 alleles for a gene. ABO blood group has 3 alleles– A allele
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IA and B allele IB are equally dominant, O allele i is recessive. Individuals have only 2 of these alleles.
Lethal alleles – result in the death of an individual that receives them. May be dominant or recessive traits.
Polygenic inheritance –occurs when a trait is controlled by more than 1 gene. Traits usually show
continuous variation such as height, skin pigmentation. Discontinuous variation is where there are
distinct phenotypes, controlled by 1 gene.
Linked genes – are genes that occur close together on the same chromosome, so do not behave
independently but are usually inherited together in the same gamete (called parental or non-crossover
gametes). If 2 genes are a long way apart there is more likely to be crossing over or recombination and
the alleles may swap chromosomes (recombinant /crossover eggs).
Test crosses can show linked genes, where the ratios for different outcomes are different to those expected.
Read additional handout material.
Pg 231 - Review Questions 21, 22, 23, 24
Biozone – 285-multiple alleles, 278-lethal alleles, 303-polygenes, 291 -linkage,
Pedigrees – use symbols to show inheritance patterns of a trait through a family. Can be used to determine
the type of inheritance involved, and predict transmission of genes to future children. Symbols - pg. 232.
Autosomal recessive inheritance – both alleles must be recessive to show trait. Both parents must be
carriers or show the trait. If both parents have the trait all offspring will have it. If both are carriers there is
25% chance of offspring showing it. Trait often skips a generation. Males & females equally affected.
Eg. Albinism, cystic fibrosis, thalassaemia, red hair colour,
Autosomal dominant inheritance – only 1 allele required to show the trait. At least 1 parent has the trait
if the offspring show it. Usually appears in every generation. 1 parent homozygous dominant – all
offspring show trait. 1 parent without trait other heterozygous - 50% chance of offspring showing it. Males
& females equally affected.
Eg. Huntington disease, achondroplasia (dwarfism), cleft palate
Sex (X) linked inheritance – occurs when an allele for a trait is located on the X chromosome. Males only
receive one copy of the X chromosome so will show the phenotype of the 1 allele they have.
Sex linked recessive traits - often masked in females as the dominant allele could be on other X.
Homozygous recessive mothers show the trait, all sons show the trait and all daughters are carriers.
Heterozygous mothers are carriers can pass traits onto sons. More males will show the trait, can skip a
generation. A man with the trait will have carrier daughters but will not affect his sons.
Eg. haemophilia, red-green colour blindness
Sex linked dominant traits – will show in females with only 1 allele for the trait, at least 1 parent must
show the trait to pass it on. A man will pass it on to all daughters, no sons. Females can pass it on to sons
& daughters. If a trait is lost from 1 branch it does not reappear. Often more females affected.
Eg. vitamin D resistant rickets
Pg 237 - Review Questions 26, 27, 28
**Biozone – 301 - inheritance patterns, 299/0-sex linked, 305/6 – pedigrees – you need to be able to
interpret pedigree diagrams and recognise inheritance patterns – learn the rules…..
Pg 238 – Visual Summary – know all terms & meanings, how to use them
Apply understanding
1, 2, 4, 6, 7, 9, 11, 12
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