Genetics (Science 1.9)
Demonstrate understanding of biological ideas relating to genetic variation
Introduction to the topic and key words
Key Words List - Learn these
Welcome to the topic of genetics! Genetics is the study of heredity. Heredity is a biological process where a parent passes certain genes onto their children or offspring. Every child inherits genes from both of their biological parents and these genes in turn express specific traits. Some of these traits may be physical for example hair and eye color and skin color etc. On the other hand some genes may also carry the risk of certain diseases and disorders that may pass on from parents to their offspring. This topic is like a new language for most so stick at it. Learn the key words, watch the videos and listen to your teachers!
Starting the topic
Fantastic PowerPoint and resources from GZScience online here
A good set of complete notes here
Before you start work through the following activities from DNA from the beginning.
No brain to small - This website has some amazing!! resources - go have a look here
The roles of DNA in both carrying instructions to the next generation and determining phenotype
Start with this animation. It will show you the main ideas of where and what DNA is.
And another one. Here
Below are the key parts to know:
Nucleus is the Organelle in a cell that contains DNA.
Deoxyribonucleic Acid is a self-replicating molecule present in nearly all living organisms. It’s what the chromosomes are made up of.
The Chromosome is a threadlike structure of nucleic acids and protein found in the nucleus of most living cells. They carrying genetic information in the form of genes. Chromosomes are made up of long lengths of DNA.
Chromosomes Location: Contained within the nucleus
Made up of: DNA (nucleic acids – a phosphate, sugar and base) with various binding proteins holding it together
Function (what it does): Containing genetic information to enable an organism to manufacture all the proteins required to develop and maintain an organism when necessary.
A Gene is a short length of DNA that carries the genetic code for a particular characteristic or cell activity. Different forms of the same gene are called Alleles. They can be dominant or recessive.
A trait is a genetically determined characteristic such as eye colour or hair colour.
Explanation of link between DNA, chromosomes and genes.
Learn Coach video
Benjamin Himme's epic video on DNA, she is a long one!
DNA is the heredity material of the cell which is found in the chromosomes in the nucleus. These are found as strands each one of these strands of DNA is called a chromosome. A gene is a segment of DNA, found in a small section of the chromosome. Along the DNA, base sequences provide the code for building different proteins, which then determine particular features. Slight differences in the sequence of the bases making up a gene are called alleles and they cause the variations in the phenotypes. These differences lead to genetic variation between individuals.
The relationship between DNA, alleles, genes, and chromosomes
From 2013's exam.. Chromosomes are made up of DNA. DNA is a large molecule that is coiled into a double helix (twisted ladder structure). It is responsible for determining the phenotype of an organism. Along this molecule are bases. These bases pair up; A always pairs with T, and G with C.
A sequence of bases which codes for a particular trait (eg, eye colour) is called a gene.
The different versions of each gene are called alleles, and these show the different variations of each characteristic, eg brown / blue eyes. Because chromosomes come in pairs for each trait, there will be two possible alleles. These different versions of genes (alleles) occur as the DNA base sequence is different.
This combination of alleles for each trait is called the genotype; this can be any combination of two of the available alleles. The genotype determines the phenotype (the physical appearance) of the organism. Whichever alleles are present may be expressed. Dominant alleles (B) will be expressed over recessive alleles (b).
So what is DNA exactly?
DNA from the Beginning Good introduction to DNA and genetics
DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).
The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.
DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.
The shape of DNA at the molecular level is thought to look like a gently twisting ladder. Each of the rungs on the ladder represents a chemical bond between the chemicals that make up the DNA molecule. These chemicals are called nucleotides and include: (click on image to make bigger)
DNA is made from Deoxyribonucleic Acid. DNA is called a polymer because it is made up of many repeating units called nucleotides.
How it is arranged
DNA strands are loose within the nucleus of a cell. Just prior to cell division the DNA folds up around proteins called histones into tight coils, then into structured chromosomes. The human cell has 46 chromosomes arranged into 23 pairs of chromosomes. Each chromosome in a pair has the same genes, called homologous pairs – except the sex chromosome pair – although there may be variation between the genes of each pair, as one comes from the father and one comes from the mother.
Chromosomes are made up of long lengths of DNA arranged in a twisted ladder.
The base pairing rules mean that guanine (G) always bonds to cytosine (C), and thymine (T) always bonds to adenine (A).
Complete the following interactive workshop http://learn.genetics.utah.edu/content/molecules/gene/
A gene is a section of DNA that carries the genetic code for a particular characteristic. An allele is an alternative form of a gene. They can be dominant or recessive.
During fertilisation a person gets two different alleles for the same gene because one allele is inherited from your mother, the other comes from your father.
A great video that covers this in more detail than is required a L1. You need to know where it happens, why it happens and the basics steps that are below.
The original DNA strand unwinds as the bonds between the bases break.
New nucleotides are brought in. They bond with the bases on the original DNA strand according to the base-pairing rules.
Once the new nucleotides have bonded, the DNA molecule begins to coil back up into a double helix. At the end of the process, two new strands of DNA are produced. Both are exact copies of the original strand.
Chromosomes come in pairs. One pair is the sex chromosomes – XX in females and XY in males. A complete set of chromosomes of an organism placed into pairs of matching chromosomes is called a karyotype. The human karyotype consists of 23 pairs of chromosomes
The way in which genotype determines phenotype
Genotype is the genetic make-up of an individual organism. Your genotype functions as a set of instructions for the growth and development of your body. The word ‘genotype’ is usually used when talking about the genetics of a particular trait (like eye colour).
Phenotype is the observable physical or biochemical characteristics of an individual organism, determined by both genetic make-up and environmental influences, for example, height, weight and skin colour.
A codon is a group of three bases that code for an specific amino acid.DNA contains the instructions for linking amino acids. These amino acids join together to make proteins. Proteins are important because they are the building blocks of our body and carry out many important functions within the body. The base sequence of DNA can be broken down into codons (three-letter sequences). One codon codes for one amino acid.
The way chromosomes exist as pairs so that individuals inherit two copies of each gene
Homologous pairs are chromosomes that have the same genes.
Biological concepts and processes relating to variation in phenotype will be selected from:
The significance of an allele as an alternative version of a gene
An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome.
Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked.
The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r).
Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked. Using the previous example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. Round: (RR) or (Rr), Wrinkled: (rr).
The role of mutations in forming new alleles
A mutation is a change in the base sequence of DNA caused by a mutagen. A mutagen is an agent, such as a chemical substance, UV light or radiation, that causes genetic mutation.
Types of mutation
Mutation is a permanent / random changes in the DNA/ genetic material. Mutation must occur in gamete-producing cells to enter the gene pool of the population
A mutation is a permanent (unrepaired) change in an organisms DNA.
They introduce new alleles into a population. Most mutations are harmful.
Mutations are caused by mutagens.
Beneficial ones tend to occur more often in organisms with short generation times.
Many may be silent – not observed – and may only be selected for or against at a later date.
Neutral mutations make no change at all.
Beneficial mutation = A mutation that gives an organism a survival advantage.
Harmful mutation = A mutation that effects the survival of the organism.
Silent mutation = a mutation which has no observable effect on the organism.
Wiki article link
Mutation animation link
The role of meiosis in generating gametes
(you are not required to provide the names of the stages of meiosis)
Did you know?
Sex cells have one set of chromosomes; body cells have two. Click here to work through an animation
Excellent animation Click here
Online simulator for Mitosis and Meiosis Click here
Meiosis is a type of cell division that occurs in the testes (males) and ovaries (females). It produces four new cells (gametes) that are genetically different to each other, and to the parent cell. They contain half the number of chromosomes that are in the parent cell.Meiosis leads to genetic variation via two processes. When homologous pairs of chromosomes line up during meiosis, they do so randomly. This means it is completely random which combination of alleles end up in a particular gamete. This process is called independent assortment.The second way meiosis leads to genetic variation is via a process called crossing over. This occurs when homologous pairs of chromosomes line up at the cell equator and swap sections of genetic material, and therefore alleles. Because of crossing over, each gamete will contain different combinations of alleles.
Mitosis explained (in more detail than you need at L1)
The significance of sexual reproduction
(in producing a new mix of alleles)
With sexual reproduction two individuals contribute genetic material with traits generally being determined by the two alleles for each gene. The process of meiosis which creates the gametes and recombination leads to an individual with a genetic make-up that differs from both parents. Over time that process allows the movements of alleles from one population to the next.
• Gametes are sex cells (sperm and egg) which are formed in the testes and ovaries. During gamete formation (meiosis), the homologous chromosomes are halved and the gamete will inherit one of each pair of chromosomes. Which chromosome is passed on is random due to the process of independent assortment.
• During fertilisation, the gametes combine and the resulting offspring will have two alleles – they may inherit two alleles the same, homozygous, and show that characteristic or they may inherit one of each allele, heterozygous in which case they will show the dominant allele in their phenotype. Genetic variation: variety within a population, eg different alleles possible for each gene. The advantage of variation to a population is that it may see some individuals survive if environment changes, in this case if drought occurs. Because of variation, not all individuals will be wiped out. Those with favourable alleles / traits / phenotypes will survive and be able to pass on genetic material to offspring and therefore survival of the species occurs.
• Possible disadvantages: need two parents that are able to reproduce, if conditions are stable could introduce variation, which may be counterproductive.
The patterns of inheritance involving simple monohybrid inheritance showing complete dominance
If you work through the student part of this animation series you will know everything you need to on monohybrid crosses.
Sex determination, possible genotypes, and phenotype ratios.
humans and many other animal species, sex is determined by specific chromosomes. How did researchers discover these so-called sex chromosomes? The path from the initial discovery of sex chromosomes in 1891 to an understanding of their true function was paved by the diligent efforts of multiple scientists over the course of many years. As often happens during a lengthy course of discovery, scientists observed and described sex chromosomes long before they knew their function.
In humans, females inherit an X chromosome from each parent, whereas males always inherit their X chromosome from their mother and their Y chromosome from their father. Consequently, all of the somatic cells in human females contain two X chromosomes, and all of the somatic cells in human males contain one X and one Y chromosome (Figure 3). The same is true of all other placental mammals — males produce X and Y gametes, and females produce only X gametes (Figure 4). In this system, referred to as the XX-XY system, maleness is determined by sperm cells that carry the Y chromosome.
The following punnet square is usually required to be completed in a sex determination question, notice you have a 50% chance of being male or female.
A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next. This is used most commonly humans, show dogs, and race horses and NCEA usually asks you to identify genotype from the pattern shown in the chart.
- Many phenotypes show continuous variation – there are many intermediate forms and the distribution of the phenotypes in a population is a bell curve.
- Traits that show continuous variation include: height and weight in humans, milk production in cows and the size of flowers.
- Continuous traits are usually polygenic, with alleles of many different genes contributing to produce the final phenotype.
- This results in considerable phenotypic variation, and further variation may also occur due to interactions with the environment.
- The phenotypes we have consider so far have been due to two segregating alleles of single genes.
- This means that alternative phenotypes are clearly different.
- This variation or difference between phenotypes is known as discontinuous variation.
•Variation in phenotype in populations is influenced by the following factors:
–One gene can have many alleles
–Meiosis involves independent assortment and genetic recombination of alleles
–Dominance may not be complete
–Several genes can affect the one trait
–One gene can determine more than one trait
–The expression of genes can be affected by other genes and by the environment
Differing rates of survival by various members of a group may depend on their phenotype
A genetically "healthy" population is defined as having a large amount of genetic variability. The information for each of an organism's characteristics is carried on a gene, but a gene can have different forms. These are known as alleles, and a large range of alleles leads to a wide variety of genetic "options" or genetic "possibilities".
With a large amount of genetic variation, natural selection is able to operate by altering the frequency of particular alleles in response to environmental conditions. Alleles providing a survival benefit to a population increase in frequency while those producing a selective disadvantage decrease or are lost altogether from the population.
The impact of natural selection acting on populations can be observed through phenotypic (physical) traits.
What is natural selection?
natural selection A good starter activity
Evolution lab. A good interactive activity
The importance of variation within populations
(population and species survival) in a changing
The advantages and disadvantages of sexual reproduction.
Sexual Reproduction is reproduction involving only two parents. The offspring are different to the parent and to each other.
• Allows for genetic variation due to the joining of two different individuals.
• Population has a greater chance of surviving if the environment changes.
• Less population growth.
• Relatively slow process.
• Requires more energy use.