Biology notes, chapter 11

Inheritance

Textbook Biology Form 5 (DLP), pages 228–253

What this chapter covers

Define monohybrid inheritance, explain Mendel's experiment on pea plants and state Mendel's First Law (11.1)
Use the terms gene, allele, trait, genotype, phenotype, dominant, recessive, homozygote, heterozygote, purebreed, hybrid, P and F generations (11.1)
Draw schematic diagrams and Punnett squares for monohybrid crosses and predict ratios (11.1)
Define dihybrid inheritance, work out the 9:3:3:1 ratio and state Mendel's Second Law (11.2)
Explain the relationship between locus, gene and allele on a pair of homologous chromosomes (11.3)
Describe autosomes, sex chromosomes and the human karyotype, and explain genetic disorders caused by nondisjunction (11.4)
Solve problems on ABO blood group, rhesus factor, thalassemia, sex determination and sex-linked inheritance (11.4)
Interpret a family pedigree (11.4)

11.1Monohybrid inheritance

Monohybrid inheritance involves the inheritance of one characteristic with contrasting traits, controlled by one gene. Genetics is the study of heredity and the variation of inherited characteristics. Gregor Mendel, the father of modern genetics, was the first to explain the principles of inheritance scientifically.

Mendel's experiments with pea plants

Mendel used the pea plant, Pisum sativum, because it grows easily and bears many seedlings, has both male and female reproductive structures, has a short generation interval and has obvious contrasting traits. He studied seven characteristics.

Table of the seven pea characteristics with dominant and recessive traits: seed shape round or constricted, seed colour yellow or green, pod shape inflated or constricted, pod colour green or yellow, flower colour purple or white, flower position axial or terminal, plant height tall or dwarf — The seven characteristics studied by Mendel.

Textbook figure 11.1, p. 230.

Mendel started with purebreed plants as the parental (P) generation. He crossed a purebreed tall plant (TT) with a purebreed dwarf plant (tt):

All the F₁ offspring were tall (Tt). The tall trait is dominant; the dwarf trait is recessive and was suppressed.
When F₁ plants were self-pollinated (Tt × Tt), the F₂ generation had tall and dwarf plants in a ratio of 3 : 1. The dwarf trait reappeared.

Terms used in inheritance

Term	Meaning	Example
Gene	Basic unit of inheritance: a DNA segment at a specific locus of a chromosome that controls a characteristic	Gene for height
Allele	An alternative form of a gene, at the same locus on a pair of homologous chromosomes	T and t
Characteristic and trait	A characteristic is a heritable feature; a trait is a variation of it	Height (characteristic); tall or dwarf (traits)
Genotype	Genetic composition of an organism, which cannot be seen	TT, Tt, tt
Phenotype	Observable characteristic of an organism	Tall, dwarf
Dominant allele	Always shows its trait when present; written as a capital letter	B
Recessive allele	Shows its trait only when both alleles are recessive; written as a small letter	b
Homozygote	Two identical alleles	BB (homozygous dominant), bb (homozygous recessive)
Heterozygote	Two different alleles	Bb
Purebreed	Carries two identical alleles for a trait; self-cross always gives offspring with the same trait	BB or bb
Hybrid	Product of mating between two purebreed varieties	Bb
Parental (P) and filial (F) generations	P is the first generation that is mated; F₁, F₂ are the successive generations	F₁ from P × P

Do not mix up the two words: characteristic is "flower colour", trait is "purple". Genotype is the letters (Bb); phenotype is what you see (purple).

A monohybrid cross step by step

Purple (BB) × white (bb) flowers

Parental genotypes: BB × bb. Gametes (by meiosis): B and b.
Fertilisation gives F₁: all Bb, all purple.
F₁ self-cross: Bb × Bb. Gametes: B, b from each parent.
F₂ genotypes: 1 BB : 2 Bb : 1 bb. F₂ phenotypes: 3 purple : 1 white.

Punnett square of Bb crossed with Bb: BB purple, Bb purple, Bb purple, bb white — Punnett square for Bb × Bb: 3 purple : 1 white.

Mendel's real ratios were close to 3 : 1 but not exact (for example 787 tall : 277 dwarf = 2.84 : 1) because fertilisation is by chance. Ratio from counts: divide every number by the smaller one. Round : constricted = 5474 : 1850 = 2.96 : 1, about 3 : 1.

Mendel's First Law (Law of Segregation)

A characteristic of an organism is controlled by a pair of alleles, and only one allele of the pair is inherited in a gamete.

The pair of alleles separates randomly during gamete formation (meiosis), so each gamete has one allele.
The zygote receives one allele from each parent, and fertilisation is random.
Homozygous dominant and heterozygous genotypes show the dominant trait; only homozygous recessive shows the recessive trait.

Two black rats (B is the allele for black fur, b for white) have offspring in the ratio ¼ BB : ½ Bb : ¼ bb. What are the genotypes of the parents?

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Both parents are Bb (heterozygous black fur). Bb × Bb gives 1 BB : 2 Bb : 1 bb, a 3 black : 1 white phenotypic ratio.

11.2Dihybrid inheritance

Dihybrid inheritance involves the inheritance of two characteristics, each controlled by a different gene at a different locus.

Cross a purebreed round, yellow pea (BBKK) with a purebreed constricted, green pea (bbkk). B is round, b constricted, K yellow, k green.

Gametes: BK and bk. F₁ is all BbKk: round and yellow (only the dominant traits show).
F₁ × F₁ (BbKk × BbKk): each parent makes four kinds of gametes, BK, Bk, bK and bk, because any allele for seed shape can pair with any allele for seed colour.
That gives 4 × 4 = 16 combinations in the Punnett square.

Punnett square of BbKk crossed with BbKk with 16 cells coloured by phenotype: nine round yellow, three round green, three constricted yellow, one constricted green — F₂ of a dihybrid cross: 9 round yellow : 3 round green : 3 constricted yellow : 1 constricted green.

F₂ phenotype	Number of boxes	Fraction
Round, yellow	9	9/16
Round, green	3	3/16
Constricted, yellow	3	3/16
Constricted, green	1	1/16

Two new combinations appear in F₂, constricted yellow and round green, which were not in the parents.

Mendel's Second Law (Law of Independent Assortment)

During gamete formation, each allele of a pair of alleles can combine randomly with any allele of another pair.

Gametes: write the allele pairs and combine one letter from each. BbKk gives BK, Bk, bK, bk. A pure line such as BBKK gives only BK. Always check that the Punnett square has 16 boxes for a dihybrid F₁ cross.

In the 16-box square, what is the probability of a round, green seed?

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3/16. The round, green boxes are BBkk (1 box) and Bbkk (2 boxes).

11.3Genes and alleles

A locus is the specific location of a gene on a chromosome. Each chromosome carries many genes; in humans about 25 000 genes that code for proteins are found in one haploid set.

An allele that represents a gene is found at the same locus on each chromosome of a pair of homologous chromosomes.
Same alleles at a locus (FF or jj): the gene is homozygous. Different alleles (Mm): heterozygous.

Analogy

Chromosome = a shopping mall; locus = a set of numbered boxes on one car park level; gene = the type of car in those boxes (sedan); allele = the model of sedan (P, Q or R).

A gene is not the same as an allele. The gene (for flower colour) sits at one locus; the alleles (P and p) are its alternative forms.

11.4Inheritance in humans

Chromosomes and karyotype

	Autosome	Sex chromosome
Number	22 pairs (numbers 1 to 22)	1 pair (number 23)
Function	Control all characteristics of somatic cells	Carry genes that determine gender
Examples	Blood group, height, skin colour	Male XY, female XX

A human somatic cell has 44 autosomes and 2 sex chromosomes (44 + XY male, 44 + XX female). A karyotype is the number and structure of chromosomes in a nucleus, arranged in homologous pairs by size, position of centromere and banding pattern.

Nondisjunction and genetic disorders

If homologous chromosomes fail to separate in anaphase I, or sister chromatids fail to separate in anaphase II, a gamete has 22 or 24 chromosomes instead of 23. Fertilisation with a normal gamete gives a zygote with 45 or 47 chromosomes.

Disorder	Chromosomes	Description
Down syndrome	47 (45 + XY or 45 + XX)	Extra chromosome 21 (trisomy 21); males and females
Turner syndrome	45 (44 + XO)	One X chromosome missing; female
Klinefelter syndrome	47 (44 + XXY)	Extra X chromosome; male with poorly developed secondary sex characteristics

ABO blood group

Blood group is controlled by one gene with three alleles (multiple alleles): I^A, I^B and I^O. A person has only two. I^A and I^B are dominant to I^O and codominant to each other: when both are present, both show (AB).

Blood group (phenotype)	Genotype
A	I^AI^A or I^AI^O
B	I^BI^B or I^BI^O
AB	I^AI^B
O	I^OI^O

Father group A (I^AI^O) × mother group B (I^BI^O)

Gametes: I^A, I^O and I^B, I^O. Children: I^AI^B (AB), I^AI^O (A), I^BI^O (B), I^OI^O (O) in the ratio 1 : 1 : 1 : 1. The chance of a child with group O is 1/4.

Blood group does not follow Mendel's simple pattern of two alleles per gene; it shows multiple alleles and codominance.

Rhesus factor and thalassemia

Rhesus factor (antigen D): Rh⁺ is dominant, Rh⁻ recessive. Rh-positive is Rh⁺Rh⁺ or Rh⁺Rh⁻; Rh-negative is Rh⁻Rh⁻. It follows Mendel's Law. Rh⁺Rh⁺ × Rh⁻Rh⁻ gives all Rh-positive; Rh⁺Rh⁻ × Rh⁻Rh⁻ gives ½ positive : ½ negative.
Thalassemia: an inherited disease caused by a gene mutation on an autosome (chromosome 11 or 16) which gives abnormal and low haemoglobin; red blood cells are smaller and paler. A carrier (thalassemia minor) has one recessive allele and no symptoms; a patient (thalassemia major) has two recessive alleles. Two carriers have a 1/4 chance of a child with thalassemia in each pregnancy. It is confirmed by a blood test.

Sex determination

Every ovum carries 22 + X. A sperm carries 22 + X or 22 + Y. The sex of the child is decided at fertilisation by the sperm: X gives a girl (44 + XX), Y gives a boy (44 + XY). The ratio of girls to boys is 1 : 1.

Sex-linked inheritance

Genes on the sex chromosomes that control characteristics other than sex are sex-linked genes. Colour blindness and haemophilia are caused by a recessive allele on the X chromosome. The Y chromosome is shorter and has few alleles, so a male shows the trait whenever his single X carries the recessive allele.

Phenotype	Male	Female
Normal	X^BY	X^BX^B
Carrier	none	X^BX^b
Colour blind	X^bY	X^bX^b

Haemophilia (blood cannot clot because of a lack of clotting factor) is written the same way with X^H and X^h.

Normal father (X^HY) × carrier mother (X^HX^h)

Gametes: X^H, Y and X^H, X^h. Children: X^HX^H (normal girl), X^HX^h (carrier girl), X^HY (normal boy), X^hY (haemophilic boy), 1 : 1 : 1 : 1. The chance of a haemophilic child is 1/4, and it is always a boy; the chance of a haemophilic son among the sons is 1/2.

A father cannot pass an X-linked allele to his son: sons receive the Y from the father and their only X from the mother. Always write X and Y in sex-linked genotypes.

Other inherited traits

Ability to roll the tongue and free earlobes are dominant traits; inability to roll the tongue and attached earlobes are recessive. They are inherited by Mendel's Law.

Family pedigree

A pedigree is a chart over several generations showing relationships and how a characteristic is inherited. Squares are males, circles are females, a horizontal line joins a married couple, and generations are numbered with Roman numerals (I, II, III). A dominant gene normally appears in every generation; a recessive gene may be hidden in some generations. If two normal parents have an affected child, the trait is recessive and both parents are heterozygous.

A woman who is heterozygous for tongue rolling (Rr) marries a man who is homozygous for tongue rolling (RR). What are the genotypes of their children?

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Gametes R, r and R. Children RR and Rr in the ratio 1 : 1. All children can roll their tongue (the allele R is dominant); none can be rr.