Leaving Cert Notes

Notes and Anki Decks for the Leaving Cert

Genetic inheritance

Definitions

Heredity - The passing of features from one generation to the next
Genetics - The study of heredity
Fertilisation - The fusion of male and female gametes to form a diploid zygote
Gametes - Haploid cells capable of fusion

Terms

Terms	Definition
Monohybrid	One trait
Bihybrid	Two traits
Homozygous	Same form (two identical alleles)
Heterozygous	Different (two different alleles)

Sex Chromosomes

We have 46 chromosomes, or 23 pairs
Autosomes are non sex chromosomes - We have 44 Autosomes (no. 1 - 22)
The other 2 chromosomes are the sex chromosomes: the X chromosome and the Y chromosome
Males have XY
Female have XX
All embryos start life as female

Male Karyotype

Female Karyotype

Alleles

Alternative forms of genes
Units that determine heritable traits
HAIR COLOUR is the GENE
BLOND, BLACK, RED are all ALLELES
CHIN is the GENE
SQUARE and ROUND are the ALLELES

Genetic Terms

Alternative forms of genes
Dominant alleles (TT - tall pea plants)
Dominant (uppercase letter) - allele that is always expressed if present
Recessive (lowercase letter) alleles (tt - dwarf pea plants)
Recessive - the allele that is prevented from being expressed by dominant allele

Phenotype

Outward appearance
Physical characteristics
Examples:
1. Tall pea plant
2. Dwarf pea plant
3. Brown eyes

Genotype = Genetic make up of an organism

TT (homozygous dominant)
tt (homozygous recessive)
Tt (heterozygous)

Non-nucleur DNA = DNA which is found in mitochondria and chloroplast which can also be inherited via the egg ovum

Most DNA is found in chromosomes in the nucleus

(Only the head of the sperm fuses with the egg so if we get DNA from mitochondria it comes from the egg)

Punnett Square

A punnett square is used to show the possible combinations of gametes
T = tall pea plant
t = dwarf plant
Phenotype of parent = Tall pea plant x Dwarf pea plant
Genotype of parent = TT x tt
Genotype of gamete =
Genotype of progeny (first generation of progeny) = Tt
Phenotype of F. = Tall plants (all)
Mendel’s first law of segregation: Genes separate during gamete formation
Gametes fuse together
F1 generation together
Genotype of F2 progeny = TT Tt Tt tt
Phenotype of F2 progeny = 3 Tall, 1 dwarf

Incomplete dominance (codominance)

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Linked Genes = Genes that are located on the same chromosomes and are therefore inherited together

Linked Genes

DiHybrid Cross = cross involving two characteristics at a time
Example: Dwarf White Pea Plant x Tall Red Pea Plant
- T = tall
- t = dwarf
- R = red
- r = white
- Genotype of Dwarf White Pea Plant = ttrr
- Genotype of Tall Red Pea Plant = TtRr or TTRR or TTRr or TtRR

Mendel’s 1st Law of Segregation

Only one member of a pair of alleles enters a gamete

Mendel's 1st Law of Segregation

Mendel’s 2nd Law of Independent Assortment

Either member of a pair of alleles has an equal chance of entering a gamete with either member of any other pair of factors

Mendel's 2nd Law of Independent Assortment

DiHybrid Cross Example with Pea Plants

Parent Phenotype: Dwarf White x Tall Red
Parent Genotype: ttrr x Tr Rr
Gametes: tr x TR Tr tR tr

Cross:

	TR	Tr	tR	tr
tr (This is crossing with the gametes on top)	TtRr	Ttrr	ttRr	ttrr

Genotype of F¹: TtRr, Ttrr, ttRr, ttrr
Phenotype of F¹: Tall Red, Tall White, Short Red, Short White
Ratio: 1 (25%) : 1 (25%) : 1 (25%) : 1 (25%)

DiHybrid Cross Example with Guinea Pigs

B = black coat
b = white coat
S = short hair
s = long hair
Phenotypes = Black coat and short hair, White coat and long hair
Genotypes = BbSs, bbss
Table:

	bs
BS	BbSs
Bs	Bbss
bS	bbSs
bs	bbss

Genotypes of F1 Progeny = BbSs, Bbss, bbSs, bbss
Phenotypes of F1 Progeny = Black coat and short hair, Black coat and long hair, white coat and short hair, white coat and long hair
Ratio = 1 : 1 : 1 : 1

Linked Gene Example

Linked Genes 2

Linked Genes = Genes that are located on the same chromosome + inherited together
Genes that are linked don’t obey the law of independent assortment (2nd law - Mendels)
Fruit flies = Drosophilia Melanogarter
- Often used in genetic crosses

Example using Fruit Flies 1

Long Wings (L) OR Vestigial (Short) Wings (l)
Wide abdomen (W) (Homozygous Dominant) OR Narrow abdomen (w) (Homozygous Recessive)
Parent Phenotype: Long Wing, Wide Abdomen x Vestigial Wing, Narrow Abdomen
Parent Genotype: LW, LW x lw, lw
Gametes: LW x lw
Cross:

Linked Genes 3

(No need to do cross when outcome is obvious)

F¹ Phenotype: LW, lw

Example using Fruit Flies 2

Parent Phenotype: Long wing wide abdomen (Heterozygous) x Long wing wide abdomen (Heterozygous)
Parent Genotype: LW, lw x LW, lw
Cross:

Linked Genes 4

F1 Genotype: LW, LW x lw, LW x LW, lw x lw, lw
F1 Phenotype: Long & Wide x Long & Wide x Long & Wide x Vestigial (Short) & Narrow

Sex Linkage

Sex Linked Genes - Genes found normally on the X chromosome (recessive gene)
Carrier - A female who has an allele for the abnormal condition but doesn’t how it

Hemophilia

Normal Blood = N
Hemophilia = n
Female = XX
Male = XY
Normal (Carrier) = Female: X^NXⁿ Male: –
Normal (Non Carrier) = Female: X^NX^N Male: ^NY^-
Hemophilia = Female: XⁿXⁿ Male: XⁿY^-

Example

Red-green colour blindness is a sex-linked trait. If a normal man has a child with a carrier woman. What is the likelihood that their first son would be colourblind?

Non-colourblindness = C
Colourblindness = c
Parent Phenotypes: Normal Male x Carrier (for colourblindness) Female
Parent Genotype: X^CY^- x X^Ccⁿ
Cross:

	X^C	Y^-
X^C (Being Crossed with Above)	X^CX^C	X^CY^-
X^c (Being Crossed with Above)	X^CX^c	X^cY^-

F¹ Genotype: X^CX^C, X^CY^-, X^CX^c, X^cY^-
F¹ Phenotype: Normal Female, Normal Male, Carrier Female, Colourblind Male
Answer: 25% Chance of Colourblindness in Children

Heredity

Heredity is the passing on of features from parents to offspring by means of genes
Also called Genetic Inheritance

Genes

A gene is a length of DNA that codes for a specific protein
It is the unit of inheritance
It is found on a chromosome
It contains instructions to make a particular protein
Characteristics are controlled by genes

Gene Expression

Not all genes are expressed
E.g. Gene for Tallness
- Organism is small - Why?
- Maybe due to poor nutrition
Characteristics (phenotype) = heredity + environment

Heredity is the passing on of features from parents to offspring by means of genes. A gene is the unit of inheritance. They are found on a chromosome. They contain instructions to make a particular protein. Gene expression is the way in which the blueprint in a gene is decoded.

Chromosome

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Chromosomes are made up of (60%) protein and (40%) DNA
Genes are arranged along the DNA of the chromosome

Chromosome Structure

Non Coding DNA

Large sections of the chromosomes don't contain genes
This DNA is called Non Coding DNA
It’s function is not yet clear
Sometimes called “Junk DNA”