Monohybrid inheritance — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

Section 1 of 24Inheritance terms

Inheritance terms

Inheritance

Inheritance the transmission of genetic information from one generation to the next.

Gene

Gene a short length of DNA on a chromosome that codes for a characteristic.

Allele

Allele a version of a gene; one allele is inherited from each parent.

Genotype

Genotype the combination of alleles that controls a characteristic.

Phenotype

Phenotype the observable characteristics of an organism.

Section 2 of 24Dominant and recessive alleles

Dominant allele

• Dominant allele expressed if present in the genotype; given a capital letter.

Recessive allele

• Recessive allele expressed only when no dominant allele is present;
  • given the lower case version of the same letter.

Hidden allele

• Hidden allele a single recessive allele remains hidden and the dominant characteristic shows.

Section 3 of 24Homozygous and heterozygous

Homozygous and heterozygous

Homozygous

Homozygous two identical alleles of a gene, either homozygous dominant or homozygous recessive.

Heterozygous

Heterozygous two different alleles of a gene.

Pure-breeding

• Pure-breeding two identical homozygous individuals breeding together always produce offspring like themselves.

Heterozygous individuals

• Heterozygous individuals are not pure-breeding, because they can pass on different alleles.

Genotype from phenotype

• Genotype from phenotype a dominant phenotype may be homozygous or heterozygous, so the genotype cannot be told by looking.

Section 4 of 24Constructing a genetic diagram

1

Choose a letter for the alleles, with clearly different capital and lower case forms.

2

Write the parental phenotypes and genotypes.

3

Write the gametes each parent can produce, one allele per gamete.

4

Draw a Punnett square with the gametes along the sides.

5

Fill in the offspring genotypes and label their phenotypes.

6

State the ratio or probability.

Section 5 of 24Common monohybrid crosses

Cross	Offspring genotypes	Phenotypes
Homozygous dominant × homozygous recessive	All heterozygous	All dominant
Heterozygous × heterozygous	1 : 2 : 1 homozygous dominant : heterozygous : homozygous recessive	3 dominant : 1 recessive
Heterozygous × homozygous recessive	1 heterozygous : 1 homozygous recessive	1 dominant : 1 recessive
Homozygous × homozygous of the same type	All the same as the parents	Pure-breeding

• Ratios are probabilities, not exact results, and small families may not match them.

Section 6 of 24Test cross

Purpose

• Purpose identifies the genotype of an organism showing the dominant phenotype.

Method

• Method cross it with a homozygous recessive individual.

All offspring dominant

• All offspring dominant the unknown parent is homozygous dominant.

Offspring about 1 : 1

• Offspring about 1 : 1 the unknown parent is heterozygous.

Section 7 of 24More from the specification

1

Mendelian inheritance of dominant genes

• Mendelian inheritance of dominant genes heterozygous individuals carry contrasting alleles of specific genes, expressing dominant phenotypic traits according to standard ratios.

2

Monohybrid Genetic Crosses and Punnett Squares

• A monohybrid cross investigates the inheritance of single-gene traits;
  • Punnett squares systematically display maternal and paternal gametic alleles to predict offspring genotypic and phenotypic ratios.

3

Homozygous versus Heterozygous States

• Homozygous individuals carry identical alleles,
  • (either homozygous dominant, e.g., BB, or homozygous recessive, e.g., bb)
• whereas heterozygous individuals carry distinct alleles (e.g., Bb).

4

Codominance in ABO Blood Group Inheritance

• Alleles I^A and I^B are codominant because both glycoproteins are simultaneously expressed on erythrocyte cell surfaces in genotype I^A I^B,
• whereas allele I^O is recessive.

5

Daughter cell

• Cell produced by cell division.

6

Blood group

• A phenotype set by alleles I-A, I-B and I-O.

7

Protein synthesis

• Making proteins from amino acids using the DNA code.

8

Mitosis

• Cell division producing genetically identical cells.

9

Protein

• A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

10

Meiosis

• Reduction division giving four genetically different haploid cells.

11

Sequence

• The order of bases in DNA or amino acids in a protein.

12

Specific

• Able to act on only one type of substrate.

Section 8 of 24Inheritance terms

Inheritance terms

Gene

• Gene a length of DNA that codes for a protein.

Allele

• Allele a version of a gene.

Dominant allele

• Dominant allele expressed even when only one copy is present; written as a capital letter.

Recessive allele

• Recessive allele expressed only when two copies are present; written in lower case.

Genotype

• Genotype the alleles an organism has, for example Tt.

Phenotype

• Phenotype the observable characteristic, for example tall.

Homozygous

• Homozygous two identical alleles, TT or tt.

Heterozygous

• Heterozygous two different alleles, Tt.

Monohybrid inheritance

Monohybrid inheritance inheritance of a single gene controlling one characteristic.

Section 9 of 24Constructing a genetic diagram

1

State the parent phenotypes and genotypes.

2

Write the gametes each can produce, one allele per gamete.

3

Draw a Punnett square with the parental gametes on the sides.

4

Fill in the offspring genotypes.

5

State the phenotype ratio and probability.

• Choose letters with an obvious difference between capital and lower case, such as A and a.

Section 10 of 24Cross of two heterozygotes

Gametes	T	t
T	TT	Tt
t	Tt	tt

Cross of two heterozygotes

Genotypes

• 1 TT : 2 Tt : 1 tt.

Phenotypes

• 3 tall : 1 short.

Probability

• Probability a short offspring has a 1 in 4 (25%) chance.

Section 11 of 24Other typical crosses

Parents	Offspring
TT × tt	All Tt (all tall)
Tt × tt	1 Tt : 1 tt (1 tall : 1 short)
TT × Tt	1 TT : 1 Tt (all tall)
tt × tt	All tt (all short)

Section 12 of 24Probability and chance

Independent events

• Independent events each fertilisation is independent, so each child of two heterozygotes has a 25% chance of being homozygous recessive.

Small samples

• Small samples actual ratios may differ widely from the expected ratio.

Large samples

• Large samples results approach the expected ratio.

Section 13 of 24Link between genes and proteins

Dominant allele

• Dominant allele usually codes for a working protein.

Recessive allele

• Recessive allele codes for a faulty or absent protein.

Heterozygote

• Heterozygote one working copy gives the normal phenotype, so the recessive allele is masked.

Recessive conditions

• Recessive conditions such as cystic fibrosis only appear with two faulty copies.

Section 14 of 24Test cross

Method

• Method cross an organism of dominant phenotype but unknown genotype with a homozygous recessive.

Any recessive offspring

• Any recessive offspring show that the unknown parent was heterozygous.

Section 15 of 24More from the specification

1

Mitosis vs Meiosis in Cells

• Mitosis produces two genetically identical diploid body cells for growth and tissue repair,
  • (such as replacing skin and blood cells)
• whereas meiosis produces four genetically diverse haploid gametes containing half the original chromosome number.

2

Genes, Chromosomes, and Protein Synthesis

• Chromosomes consist of long DNA molecules carrying genes,
• which provide the triplet code that determines the sequence of amino acids in synthesized proteins.

3

Monohybrid Inheritance and Phenotypes

• In an individual organism, alleles located on homologous chromosomes determine phenotypic traits according to dominant, recessive,
• or codominant expression patterns.
  • (such as ABO blood groups)

4

Daughter cell

• Cell produced by cell division.

5

Blood group

• A phenotype set by alleles I-A, I-B and I-O.

6

Protein synthesis

• Making proteins from amino acids using the DNA code.

7

Chromosome

• A thread-like structure of DNA carrying genes.

8

Mitosis

• Cell division producing genetically identical cells.

9

Meiosis

• Reduction division giving four genetically different haploid cells.

10

Sequence

• The order of bases in DNA or amino acids in a protein.

11

Specific

• Able to act on only one type of substrate.

Section 16 of 24Sex determination

Sex chromosomes

• Sex chromosomes one of the 23 pairs; females are XX and males are XY.

Eggs

• Eggs always carry an X.

Sperm

• Sperm carry either X or Y, so the sperm decides the sex of the child.

Gametes	X (egg)	X (egg)
X (sperm)	XX	XX
Y (sperm)	XY	XY

• Probability
  • 50% for each sex at every birth.

Section 17 of 24Pedigree charts

Pedigree charts

Pedigree chart

• Pedigree chart a family tree showing inheritance of a characteristic through generations.

Symbols

• Symbols squares are males, circles are females and shaded symbols are affected people.

Two unaffected parents with an affected child

• Two unaffected parents with an affected child the condition is recessive and both parents are carriers.

Condition in every generation

• Condition in every generation with affected children having an affected parent, it may be dominant.

Checking

• Checking write the possible genotypes and test them with a Punnett square.

Section 18 of 24Inherited conditions

Condition	Allele	Effect
Cystic fibrosis	Recessive	Thick sticky mucus in the lungs and gut
Sickle cell anaemia	Recessive allele of the haemoglobin gene	Red blood cells become sickle shaped and carry less oxygen
Huntington's disease	Dominant	Symptoms appear in middle age

Cystic fibrosis

• Cystic fibrosis two carriers have a 1 in 4 chance of an affected child.

Sickle cell carriers

• Sickle cell carriers heterozygotes have some protection against malaria, so the allele is common where malaria occurs.

Section 19 of 24Reasoning from a pedigree

1

Unaffected parents with an affected child prove the condition is recessive.

2

Affected people are homozygous recessive and their parents are carriers.

3

Write the cross and the gametes.

4

Read the probability from the Punnett square.

Reasoning from a pedigree

Carrier

• Carrier a person heterozygous for a recessive harmful allele.

Unaffected child of two carriers

• Unaffected child of two carriers has a 2 in 3 chance of being a carrier.

Independence

• Independence the probability for the next child does not depend on earlier children.

Section 20 of 24Codominance and sex-linked genes

1Codominance

• Codominance both alleles are expressed in a heterozygote;
  • I^A and I^B are codominant and both dominant to I^O, so I^AI^B gives group AB.

2Sex-linked genes

• Sex-linked genes carried on the X chromosome.

3Red-green colour blindness

• Red-green colour blindness a recessive allele on the X;
  • males have one X, so one allele is enough, making it more common in males.

4Carrier mother and normal father

• Carrier mother and normal father a 50% chance that a son is colour blind and no daughter is affected.

Section 21 of 24Genetic counselling

Counselling

• Counselling uses pedigrees and probability to advise families.

Embryo screening

• Embryo screening raises ethical questions.

Section 22 of 24More from the specification

1

Monohybrid Genetic Crosses

• Punnett squares predict the genotypes and phenotypes of offspring resulting from the combination of alleles from two parent individuals.
  • (e.g. tall versus short phenotypes)

2

Meiosis vs Mitosis

• Meiosis is a reduction division producing four haploid, genetically varied gametes,
• whereas mitosis is a cell division that yields two genetically identical diploid cells.

3

Genes, Alleles, and Phenotype

• A gene is a specific sequence of nucleotides encoding an amino acid sequence in a protein,
• which determines the observable physical phenotype of an individual.

4

Codominance and Incomplete Dominance

• When both alleles contribute equally to the phenotype of a heterozygous individual, neither allele masks the other completely.

5

Protein synthesis

• Making proteins from amino acids using the DNA code.

6

Mitosis

• Cell division producing genetically identical cells.

7

Meiosis

• Reduction division giving four genetically different haploid cells.

8

Sequence

• The order of bases in DNA or amino acids in a protein.

9

Phenotype

• The observable characteristic.

10

Specific

• Able to act on only one type of substrate.

11

Haploid

• Having half the normal number of chromosomes.

Section 23 of 24Key terms

Term	Meaning
Allele	A version of a gene.
Dominant	Expressed when present in the genotype.
Recessive	Expressed only when no dominant allele is present.
Homozygous	Having two identical alleles.
Heterozygous	Having two different alleles.
Genotype	The alleles an organism has.
Phenotype	The observable characteristic.
Punnett square	A grid predicting offspring genotypes.

Term	Meaning
Dominant allele	An allele expressed when present.
Recessive allele	An allele expressed only when homozygous.
Genotype	The alleles an organism has.
Phenotype	The observable characteristic.
Monohybrid inheritance	Inheritance of one gene.
Test cross	Crossing with a homozygous recessive to find a genotype.

Term	Meaning
Pedigree chart	A family tree showing inheritance of a characteristic.
Carrier	A person heterozygous for a recessive allele who is unaffected.
Cystic fibrosis	A recessive condition causing thick sticky mucus.
Sickle cell anaemia	A recessive condition in which red blood cells are sickle shaped.
Huntington's disease	A dominant condition with symptoms in middle age.
Genetic counselling	Advice to families about the chances of inherited conditions.

Section 24 of 24Exam technique

1

Saying a recessive allele cannot be passed on — heterozygotes pass it on.

2

Mixing up genotype and phenotype — genotype is the alleles and phenotype is the observable feature.

3

Writing gametes with two alleles — each gamete carries one allele.

4

Saying a 3 : 1 ratio is guaranteed in a small family — it is only a probability.

1

Using letters that look alike in capital and lower case, like C and c — use A and a.

2

Saying previous children affect the next child's chances — each fertilisation is independent.

3

Writing two alleles in one gamete — each gamete has one.

4

Saying a recessive allele is lost in a heterozygote — it is masked, not lost.

1

Saying a recessive condition needs only one parent to carry it — both parents must carry the allele.

2

Starting a pedigree with unknown individuals — begin with the recessive individuals, whose genotype is known.

3

Saying the sperm and egg both decide sex — only the sperm decides.

4

Saying carriers have the condition — they have one recessive allele and are unaffected.